fix(ci): use correct CALENS_* secrets for changelog workflow

[DeepDiver1975]

Description

The changelog workflow (.github/workflows/changelog.yml) calls the reusable owncloud/reusable-workflows/.github/workflows/calens.yml@main, which requires the secrets APP_ID and APP_PRIVATE_KEY. This PR updates the source secrets mapped into those inputs:

• CHANGELOG_APP_ID → CALENS_APP_ID
• CHANGELOG_APP_PRIVATE_KEY → CALENS_APP_PRIVATE_KEY

Motivation and Context

The org standardized the GitHub App secrets used by the changelog automation on the CALENS_* names. owncloud/web and owncloud/client already map APP_ID: ${{ secrets.CALENS_APP_ID }} / APP_PRIVATE_KEY: ${{ secrets.CALENS_APP_PRIVATE_KEY }}, and owncloud/core was the only repo still referencing the old CHANGELOG_* names — leaving its changelog automation unable to authenticate. This brings core in line with the rest of the org.

The reusable workflow's contract is unchanged: only the right-hand side (the source secret names) is updated; the required APP_ID / APP_PRIVATE_KEY keys remain intact.

How Has This Been Tested?

The token-dependent steps in the reusable workflow are gated by if: github.ref == format('refs/heads/{0}', inputs.branch), so they only run on master, not on this PR. To be verified via the first changelog run on master after merge.

Prerequisite: the CALENS_APP_ID / CALENS_APP_PRIVATE_KEY secrets must be provisioned for this repo (org-level secret with owncloud/core in its repository-access allowlist, or repo-level secrets). This is very likely already satisfied since web/client use the same names.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised: n/a
• Changelog item — n/a (CI-only change)

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.