Add Trino Gateway 18 release notes

[mosabua]

Description

Assemble the release notes for Trino Gateway 18 release and adjust rest of docs to version 18.

Release date will be to the planned date following our monthly release cadence after dev sync discussion and agreement

Additional context and related issues

• Helm chart release PR to merge after the release to be done

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.

Verification for each pull request

Format: PR/issue number, ✅ / ❌ rn ✅ / ❌ docs
✅ rn - release note added and verified, or assessed to be not necessary, set to ❌ rn before completion
✅ docs - need for docs assessed and merged, or assessed to be not necessary, set to ❌ docs before completion

Any dates missing in the list just had no merged PRs.

1 Feb 2026

• Bump actions/setup-java from 4 to 5 #886 ✅ rn ✅ docs
• Bump github/lock from 2 to 3 #885 ✅ rn ✅ docs
• Bump actions/checkout from 4 to 6 #884 ✅ rn ✅ docs
• Migrate from Guava Cache to Caffeine #808 ✅ rn ✅ docs