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/values.yaml

@@ -206,8 +206,8 @@ minecraft:
   # This value must match the Java line of the chart version: the image tag
   # comes from .Chart.AppVersion, so the JVM is fixed per published chart
   # version. Java 8 runs Minecraft up to 1.16.5, 1.18-1.20.4 need Java 17 and
-  # 1.20.5+ need Java 21. This chart publishes appVersion "2026.8.2-java17".
-  version: "1.20.1"
+  # 1.20.5+ need Java 21. This chart publishes appVersion "2026.8.2-java21".
+  version: "1.21.1"
 
 # curseforge, ftb and curseforgeServerpack are mutually exclusive - enabling
 # more than one is rejected by the chart (before 1.0.0 it silently rendered a