Use the new "Prepare release" GitHub Actions workflow (#536)

We now have a new reusable GitHub Actions workflow for preparing
a new classic buildpack release:
https://github.com/heroku/languages-github-actions/blob/main/.github/workflows/_classic-buildpack-prepare-release.yml

This workflow works the same as the CNB version, except it does
not need any version bump input, since we only every increment 
classic buildpack versions by one.

GUS-W-14901966.

.github/workflows/prepare-release.yml

@@ -0,0 +1,12 @@
+name: Prepare Release
+
+on:
+  workflow_dispatch:
+
+# Disable all GITHUB_TOKEN permissions, since the GitHub App token is used instead.
+permissions: {}
+
+jobs:
+  prepare-release:
+    uses: heroku/languages-github-actions/.github/workflows/_classic-buildpack-prepare-release.yml@latest
+    secrets: inherit