[GLUTEN-12412][VL] Migrate x86/arm vcpkg native lib build from CentOS 7 to AlmaLinux 8

[KushShriv]

What changes are proposed in this pull request?

Fixes #12412 (2)

CentOS 7 (glibc 2.17) is increasingly difficult to maintain as a vcpkg build base due to Velox's growing dependency on modern Folly/FBThrift/MVFST versions. This PR migrates the x86/arm native lib CI jobs to use the AlmaLinux 8 (glibc 2.28) image published in the prerequisite PR, completing the transition away from CentOS 7.

Migrate x86/arm native lib build from CentOS 7 to AlmaLinux 8:

• velox_backend_x86.yml: rename build job centos-7 → centos-8, switch image and build script, update ccache keys and artifact names, remove the now-unnecessary tpc-test-centos7 job and CentOS mirror fixup steps.
• velox_backend_enhanced.yml, velox_backend_ansi.yml, velox_nightly.yml, velox_backend_cache.yml, build_bundle_package.yml, build_release.yml: same image/script/ccache/artifact migration.
• cpp_clang_tidy.yml: update artifact download name to match the renamed artifact from velox_backend_x86.yml.
• dev/vcpkg/README.md: update "Build in docker" quick-start to reference the new image and script.

How was this patch tested?

CI will validate all workflow changes against the AlmaLinux 8 image published in #12757.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Sonnet 4.5

[zhouyuan]

@KushShriv, thanks for the patch! Would you mind splitting it and adding the new Docker image as the first step?

[KushShriv]

@zhouyuan Here's the Other PR with just the new docker image: #12757