Add a Prepare Release GitHub Actions workflow

Add a GitHub Actions workflow for preparing a new buildpack release,
similar to that used for CNBs and `libcnb.rs` releases:
https://github.com/heroku/languages-github-actions/blob/main/.github/workflows/_buildpacks-prepare-release.yml
https://github.com/heroku/libcnb.rs/blob/main/.github/workflows/prepare-release.yml

The changelog format has been updated in this PR to more closely match the
https://keepachangelog.com format (which allows reusing the regexes from
our other automation), however, to reduce churn I've not adopted the change
categories headings for now.

For now this is not being implemented as a shared workflow, since the
classic buildpacks (unlike the CNBs):
- do not have standardised changelog formats
- have differing approaches to versioning (some have the changelog version
  match the buildpack registry version, others do not)
- in some cases (eg Ruby) require additional edits for releases beyond just the
  changelog (such as updating version constants elsewhere in the codebase).

As such, it makes more sense to get something working for one buildpack first,
and if there is interest about adding this to other classic buildpacks, we can
discuss how we might standardise/implement generic automation.

GUS-W-14437037.

.github/workflows/prepare-release.yml

@@ -0,0 +1,76 @@
+name: Prepare release
+
+on: workflow_dispatch
+
+jobs:
+  prepare-release:
+    name: Prepare Release
+    runs-on: pub-hk-ubuntu-22.04-small
+    steps:
+      - name: Get token for GH application (Linguist)
+        uses: heroku/use-app-token-action@main
+        id: generate-token
+        with:
+          app_id: ${{ vars.LINGUIST_GH_APP_ID }}
+          private_key: ${{ secrets.LINGUIST_GH_PRIVATE_KEY }}
+
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          # We always want the version bump/changelog and resultant PR to target main, not the branch of the workflow_dispatch.
+          ref: main
+          # Using the GH application token here will configure the local git config for this repo with credentials
+          # that can be used to make signed commits that are attributed to the GH application user
+          token: ${{ steps.generate-token.outputs.app_token }}
+
+      - name: Determine existing published version
+        id: existing-version
+        # This uses the buildpack registry API directly instead of the Heroku CLI, since the latter
+        # requires being logged in for version queries even though the registry API itself doesn't.
+        run: |
+          URI_ENCODED_BUILDPACK_NAME='heroku%2Fpython'
+          VERSION=$(
+            curl --silent --show-error --fail --retry 3 --retry-connrefused --connect-timeout 10 \
+                -H 'Accept: application/vnd.heroku+json; version=3.buildpack-registry' \
+                "https://buildpack-registry.heroku.com/buildpacks/${URI_ENCODED_BUILDPACK_NAME}/revisions" \
+              | jq --exit-status --raw-output 'max_by(.release) | .release'
+          )
+          echo "version=${VERSION}" >> "${GITHUB_OUTPUT}"
+
+      - name: Calculate new version
+        id: new-version
+        run: echo "version=$((${{ steps.existing-version.outputs.version }}+1))" >> "${GITHUB_OUTPUT}"
+
+      - name: Update changelog
+        run: |
+          EXISTING_VERSION='${{ steps.existing-version.outputs.version }}'
+          NEW_VERSION='${{ steps.new-version.outputs.version }}'
+          DATE_TODAY="$(date --utc --iso-8601)"
+          UNRELEASED_URL="https://github.com/${{ github.repository }}/compare/v${NEW_VERSION}...HEAD"
+          NEW_VERSION_URL="https://github.com/${{ github.repository }}/compare/v${EXISTING_VERSION}...v${NEW_VERSION}"
+
+          sed --in-place --regexp-extended \
+            --expression "s~(^## \[Unreleased\])$~\1\n\n\n## [v${NEW_VERSION}] - ${DATE_TODAY}~" \
+            --expression "s~(^\[unreleased\]:) .*$~\1 ${UNRELEASED_URL}\n[v${NEW_VERSION}]: ${NEW_VERSION_URL}~" \
+            CHANGELOG.md
+    
+      - name: Create pull request
+        id: pr
+        uses: peter-evans/create-pull-request@v5.0.2
+        with:
+          token: ${{ steps.generate-token.outputs.app_token }}
+          title: Prepare release v${{ steps.new-version.outputs.version }}
+          body: |
+            Changes:
+            https://github.com/${{ github.repository }}/compare/v${{ steps.existing-version.outputs.version }}...main
+          commit-message: Prepare release v${{ steps.new-version.outputs.version }}
+          branch: prepare-release
+          delete-branch: true
+          committer: ${{ vars.LINGUIST_GH_APP_USERNAME }} <${{ vars.LINGUIST_GH_APP_EMAIL }}>
+          author: ${{ vars.LINGUIST_GH_APP_USERNAME }} <${{ vars.LINGUIST_GH_APP_EMAIL }}>
+
+      - name: Configure pull request
+        if: steps.pr.outputs.pull-request-operation == 'created'
+        run: gh pr merge --auto --squash "${{ steps.pr.outputs.pull-request-number }}"
+        env:
+          GH_TOKEN: ${{ steps.generate-token.outputs.app_token }}