[9.18] fix: doc: Remove 9.21-only release note from May 2026 release notes

Issue #5826 has two different fixes: one released in April 2026 that
applies to 9.20 and 9.18, and another released in May 2026 that applies
to 9.21. The 9.21 release note was mistakenly included in the release
notes for 9.20 and 9.18. This commit removes it.

Backport of MR !12067

Merge branch 'backport-andoni/fix-doc-duplicate-note-5826-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12078