ci: namespace ccache by PostgreSQL major version

Hi,

When the Postgres major version is updated, CI fails with [1]:

2026-07-02 17:01:01.938 UTC [828] FATAL:  incompatible library
"D:/a/postgresql/postgresql/build/tmp_install/usr/local/pgsql/lib/utf8_and_win.dll":
version mismatch
2026-07-02 17:01:01.938 UTC [828] DETAIL:  Server is version 20,
library is version 19.

This happens because the GitHub Actions ccache is restored across a
version bump, so objects built against the previous version can be
reused, producing libraries that no longer match the server version.

Here is an attempt to solve this problem by namespacing ccache by
Postgres major version. I added a 'PG_MAJOR_VERSION' variable to the
CI file and used that as a prefix to ccache key. I made this
'PG_MAJOR_VERSION' variable automatically updated by
'version_stamp.pl' script.

Another solution could be reading the version from build files (e.g
meson.build), but then this read needs to be done at each CI run.

[1] https://github.com/postgresql-cfbot/postgresql/actions/runs/28606913807/job/84829900634

-- 
Regards,
Nazir Bilal Yavuz
Microsoft