Update minecraft-server to the java21 line (3.0.0+up2026.8.2.java21)

Last of the three publishes that carry the checksum annotations from
3.0.0+up2026.8.2.java8, leaving main on the line of the only running server.
Same chart content as the java8 and java17 publishes, 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).

Against the state before this series (2.0.0+up2026.8.2.java21) the only
replaced line in Chart.yaml and values.yaml is the chart version; appVersion
and the minecraft.version default are byte-identical, everything else is
additive.

Refs #82.

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: 3.0.0+up2026.8.2.java17
-appVersion: "2026.8.2-java17"
+version: 3.0.0+up2026.8.2.java21
+appVersion: "2026.8.2-java21"
 
 maintainers:
   - name: jklein

minecraft-server/values.yaml

@@ -226,8 +226,8 @@ 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