[3.15] gh-152785: Upgrade Ubuntu from 24.04 to 26.04 in GitHub Actions (#152717)...

[3.15] gh-152785: Upgrade Ubuntu from 24.04 to 26.04 in GitHub Actions (#152717) (#153416)

gh-152785: Upgrade Ubuntu from 24.04 to 26.04 in GitHub Actions (#152717)

* Replace "ubuntu-24.04" with "ubuntu-26.04".
* Replace "ubuntu-latest" with "ubuntu-26.04" for Cross build Linux.
* Replace "ubuntu-latest" with "ubuntu-slim" for small workloads.
* Update ".github/actionlint.yaml" to allow "ubuntu-26.04"
  and "ubuntu-26.04-arm" images.
* Install Ubuntu libmpdec-dev package, rather than installing libmpdec
  from source (tarball).
* No longer run https://apt.llvm.org/llvm.sh, since ubuntu-26.04 provides
  clang-21 by default.

(cherry picked from commit 4c79929705eb31959a6f4071cc34583de0737a28)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>