ci: namespace ccache by PostgreSQL major version

Applying: ci: Make stable branch ccache usable by branches based on them
Using index info to reconstruct a base tree...
M	.github/workflows/pg-ci.yml
Falling back to patching base and 3-way merge...
Auto-merging .github/workflows/pg-ci.yml
CONFLICT (content): Merge conflict in .github/workflows/pg-ci.yml
Patch failed at 0001 ci: Make stable branch ccache usable by branches based on them

error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"