Description of the change

This updates:

• the Stackage snapshot to LTS-20.26
  • requires happy to be updated to 1.20.1.1 from 1.20.0
• GHC to 9.2.8
• (internal) STACK_VERSION to 2.15.1
  • In the Linux CI, this needs to be updated according to whatever the Haskell image ships

Like #4433, this upgrade is done to keep up with the "recommended" versions of tooling. In particular, 9.2.8 is the latest version of GHC 9.2.x that's still supported by HLS.

Checklist:

• Added a file to CHANGELOG.d for this PR (see CHANGELOG.d/README.md)
• Added myself to CONTRIBUTORS.md (if this is my first contribution)
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)