docs: clarify RPM package links after UBI 9 migration

[ruanweihong-aaa]

Summary

• Update the Chinese offline RPM example to the verified APISIX 3.19.0 package under the EL9 x86_64 repository path.
• Explain the UBI 8 to UBI 9 migration starting with APISIX 3.14.0 in both installation guides.
• Make the versioned yum example query packages available for the current system before choosing a version. The repository URL uses $releasever, so an EL8 system cannot install 3.19.0 from its EL8 repository.
• Remove the outdated CentOS 7/8-only wording from the Chinese guide.

Fixes #13172.

Verification

• Confirmed the EL9 3.19.0 RPM URL returns HTTP 200 and the same filename under the EL8 path returns HTTP 404.
• Checked that apache-apisix.repo selects the repository path using $releasever.
• markdownlint-cli@0.25.0 passed for both installation guides after the review fix.
• git diff --check passed.

The RPM installation steps were not run on a Red Hat/UBI host.

[Baoyuantop]

On EL8, yum install apisix-3.19.0 cannot find the package. The configured repository resolves to the EL8 branch, which contains packages only through 3.13.0.

[ruanweihong-aaa]

Confirmed from the repository metadata: apache-apisix.repo uses $releasever; the EL8 x86_64 index lists apisix only through 3.13.0, while the EL9 x86_64 index lists 3.15.0 through 3.19.0. The 3.19.0 RPM URL returns 404 under /redhat/8/ and 200 under /redhat/9/. I replaced the pinned yum install apisix-3.19.0 example with a query for versions available in the current repository, and labeled the direct 3.19.0 RPM example as EL9 x86_64 (3e92361).

I also tested the installation on a Rocky Linux 9 x86_64 container on an EL9 cloud host. dnf --showduplicates list apisix listed 3.19.0. The first dnf install apisix-3.19.0 failed because Rocky's CRB repository was disabled and libyaml-devel was unavailable. After enabling CRB, the install completed; rpm -q apisix returned apisix-3.19.0-0.ubi9.6.x86_64, and apisix version returned 3.19.0. I added the Rocky Linux 9 prerequisite to both installation guides in 95f551e.