ci: use Meson's new `--slice` option

As executing our test suite is notoriously slow on Windows we use matrix
jobs in our CI systems to slice up tests and run them via multiple jobs.
On Meson this is done with a comparatively complex PowerShell invocation
as Meson didn't yet have a native way to slice tests like this.

I have upstreamed a new `--slice` option [1] that addresses this use
case though, which has been merged and released with Meson 1.8. Both
GitLab and GitHub CI have Meson 1.8.2 available by now, so let's update
the jobs to use that new option.

[1]: https://github.com/mesonbuild/meson/pull/14092

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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