Use the new "Prepare release" GitHub Actions workflow (#683)

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.

Should any custom file changes be required as part of the PHP classic
buildpack release process in the future, a bash command/commands can
be passed via the workflow's `custom_update_command` option.

GUS-W-14905318.

.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