DOC-3407: Remove placeholder Known Issues section from 8.4.0 release notes (#4047)

The Known Issues section contained unfilled template text and was
published to the live site. Removed the section, its overview link,
and the keyword entry since there are no known issues for 8.4.0.

Author: kemister85

modules/ROOT/pages/8.4.0-release-notes.adoc

@@ -2,7 +2,7 @@
 :release-version: 8.4.0
 :navtitle: {productname} {release-version}
 :description: Release notes for {productname} {release-version}
-:keywords: releasenotes, new, changes, bugfixes, new features, improvements, changes, removals, deprecated, known issues
+:keywords: releasenotes, new, changes, bugfixes, new features, improvements, changes, removals, deprecated
 :page-toclevels: 1
 
 include::partial$misc/admon-releasenotes-for-stable.adoc[]
@@ -21,7 +21,6 @@ include::partial$misc/admon-releasenotes-for-stable.adoc[]
 * xref:changes[Changes]
 * xref:bug-fixes[Bug fixes]
 * xref:security-fixes[Security fixes]
-* xref:known-issues[Known issues]
 
 
 [[new-premium-plugin]]
@@ -385,16 +384,3 @@ A link:https://owasp.org/www-community/attacks/xss/[cross-site scripting] (XSS)
 GHSA: link:https://github.com/advisories/GHSA-v2wj-7wpq-c8vv[GitHub Advisory]
 
 CVE: link:https://nvd.nist.gov/vuln/detail/CVE-2026-0540[CVE-2026-0540]
-
-
-[[known-issues]]
-== Known issues
-
-This section describes issues that users of {productname} {release-version} may encounter and possible workarounds for these issues.
-
-The following known issues affect {productname} {release-version}.
-
-=== <TINY-vwxyz 1 changelog entry>
-// #TINY-vwxyz1
-
-// CCFR here.