docs: Update supported versions

By SDP 26.3, this should no longer be rc2

Author: NickLarsenNZ

docs/modules/superset/partials/supported-versions.adoc

@@ -2,6 +2,7 @@
 // This is a separate file, since it is used by both the direct Superset documentation, and the overarching
 // Stackable Platform documentation.
 
+- 6.0.0-rc2 (experimental)
 - 4.1.4 (LTS)
 - 4.1.2 (deprecated)
 - 4.0.2 (deprecated)