Update minecraft-server to the java21 line (2.0.0+up2026.8.2.java21)

Last of the three stage-3 publishes, leaving main on the line of the only
running server. Same chart content as 2.0.0+up2026.8.2.java8 and .java17, only
the published Java line changes: appVersion moves to the dated tag
2026.8.2-java21 and the minecraft.version default moves to 1.21.1, the version
ftb-skies-2 actually runs (Java 8 runs up to 1.16.5, 1.18-1.20.4 need Java 17,
1.20.5+ need Java 21).

minecraft-server/Chart.yaml

@@ -13,8 +13,8 @@ type: application
 # continuously, so a pod restart could pull a different image than the last one
 # without anything being recorded anywhere. The dated tags are immutable, so the
 # image is now pinned by the chart version like every other chart in this repo.
-version: 2.0.0+up2026.8.2.java17
-appVersion: "2026.8.2-java17"
+version: 2.0.0+up2026.8.2.java21
+appVersion: "2026.8.2-java21"
 
 maintainers:
   - name: jklein