Deploy docker images to GHCR instead of Docker Hub

[ThomasPerkins1123]

We are sunsetting the CBMC images on Docker Hub, so release images are now
published to the GitHub Container Registry (GHCR) instead. Publishing uses the
built-in GITHUB_TOKEN, so no Docker Hub credentials are needed.

Changes

• .github/workflows/release-packages.yaml: the docker job now pushes
  ghcr.io/diffblue/cbmc:, authenticating to ghcr.io via
  GITHUB_TOKEN/github.actor and declaring packages: write. The
  DOCKERHUB_ACCESS_DB_CI_CPROVER secret is no longer used.
• .github/workflows/regular-release.yaml: release notes now point at docker
  run -it ghcr.io/diffblue/cbmc:.
• RELEASE_PROCESS.md: updated to reference the GHCR package page.

⚠️ Action required if you pull CBMC images from Docker Hub

After this PR is merged, switch to GHCR:

docker pull ghcr.io/diffblue/cbmc:<version> # was: diffblue/cbmc:

Pulling from Docker Hub will still work until 31 October 2026.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.83%. Comparing base (a777c7b) to head (8109070).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9143   +/-   ##
========================================
  Coverage    80.83%   80.83%           
========================================
  Files         1715     1715           
  Lines       189989   189989           
  Branches        73       73           
========================================
+ Hits        153576   153580    +4     
+ Misses       36413    36409    -4     

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