DOC-3243: Update TinyMCE 8.4 release and Premium Support end dates

Set 8.4 release date to 2026-03-31 and end of Premium Support to 2027-09-30 (18 months).

Author: kemister85

modules/ROOT/partials/misc/supported-versions.adoc

@@ -6,7 +6,7 @@ Supported versions of {productname}:
 [cols="^,^,^",options="header"]
 |===
 |Version |Release Date |End of Premium Support
-|8.4 |2026-02-04 |2027-02-04
+|8.4 |2026-03-31 |2027-09-30
 |8.3 |2025-12-10 |2027-06-10
 |8.2 |2025-10-22 |2027-04-22
 |8.1 |2025-09-17 |2027-03-17