[kayx23]

Why this PR is needed

GitHub now routes ubuntu-latest jobs to Ubuntu 26.04. The release/3.18 branch predates that runner image, so its pull requests fail during environment setup before their tests execute: Python 3.9 is unavailable, libpcre3 packages were removed, and apt-key no longer exists.

Master fixed the migration in #14070, and the equivalent release/3.19 backport in #14086 has passed the complete CI matrix. This PR applies the same compatibility work to release/3.18.

Changes

• use Python 3.12 for Link Checker
• remove unavailable libpcre3 packages and link lrexlib-pcre against the APISIX runtime PCRE
• replace apt-key with an /etc/apt/keyrings signed-by configuration
• fall back to the Ubuntu 24.04 OpenResty repository when 26.04 packages are unavailable
• update lua-resty-saml to 0.2.7 while retaining the branch's other dependency versions
• update the CI runtime from 1.3.16 to 1.3.18 to avoid the Dubbo worker crash fixed in the maintained runtime
• pin the CI bootstrap to pnpm 11.25.0 because pnpm 12 rejects the branch's root-run setup and blocks all test shards before execution
• preserve shard environment variables with sudo-rs
• update the OpenWhisk test service for the current Docker API floor and supported Node.js runtime
• make disk cleanup tolerate packages absent from the runner image
• include the runner image in dependency cache keys
• document Ubuntu 26.04 as supported
• synchronize the garbage-collection and stream-metrics tests with their background worker and refresh intervals
• wait for the Admin API route update to reach proxy workers before the access-log test exercises the route

The test changes retain all assertions; they prevent slower runners from starving the worker-events broker or scraping before the configured metrics refresh.

Validation

• all modified workflow YAML files parse
• bash -n passes for all modified shell scripts
• git diff --check passes
• the conflict resolution preserves the release/3.18 dependency set except for the required lua-resty-saml update

The complete PR CI matrix is the end-to-end validation for the backport.