Add CNB update workflow

[runesoerensen]

This workflow automates the process of keeping the DOTNET_CNB_VERSION in bin/compile and the CHANGELOG.md file up-to-date with new releases in the heroku/buildpacks-dotnet repository:

1. Extracts the currently pinned DOTNET_CNB_VERSION from bin/compile.
2. Queries the heroku/buildpacks-dotnet repository to find the next release following the currently pinned version.
3. Updates files in this repository:
   • Updates bin/compile to use the next CNB release.
   • Inserts the relevant release notes from the heroku/buildpacks-dotnet repository under the ## [Unreleased] section of CHANGELOG.md.
4. Creates a PR titled Update heroku/buildpacks-dotnet to v{next-cnb-release-version} with a link to the corresponding release.

The workflow is executed on a schedule, but can also be triggered manually to allow multiple CNB version updates prior to executing the "Prepare Release" workflow.

Testing this type of workflow is a bit tricky, but should work fine for common/regular version updates. There are several areas we may want to improve in the future, such as automatically triggering this workflow following successful execution of https://github.com/heroku/buildpacks-dotnet/blob/main/.github/workflows/release.yml.

Maintainers may need to edit CHANGELOG.md manually (particularly to remove entries that are not relevant for this buildpack, e.g. changes to caching logic etc).

[edmorley]

Yey automation! 😄