build: Bump Meson dependency to 1.2.0

And update all the CI builds to use the latest micro release from that series, 1.2.3.

This version bump means we can:

• Drop some backwards-compatibility Meson checks
• Fix a periodic CI failure caused by a now-fixed Meson bug (https://github.com/mesonbuild/meson/pull/10633)

It’s in line with our Meson version policy, as Meson 1.2.1 is available in Debian Trixie and the freedesktop SDK.

Signed-off-by: Philip Withnall philip@tecnocode.co.uk