Add Trino Gateway 20 release notes

[mosabua]

Description

Assemble the release notes for Trino Gateway 20 release and adjust rest of docs to version 20.

Release date will be set 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 at Release Trino Gateway chart 1.20.0 charts#425

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.

11 May 2026

• Correct property name in release notes for version 19 #1066 ✅ rn ✅ docs

12 May 2026

• Bump dep.trino.version from 480 to 481 #1067 ✅ rn ✅ docs