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

[3.14] gh-152785: Upgrade Ubuntu from 24.04 to 26.04 in GitHub Actions (#152717) (#153416) (#153526)

* [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" on most jobs.
* 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.

Backport changes:

* Keep Ubuntu 24.04 and llvm.sh to install LLVM 19 and LLVM 20.
* WASI uses ubuntu-26.04, not ubuntu-26.04-arm.

(cherry picked from commit 4c79929705eb31959a6f4071cc34583de0737a28)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
(cherry picked from commit da999e638c03a05ed2b0f8a828f4013c7f9179bd)

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