Update minecraft-server to the java21 line (1.0.0+upjava21)

PR C of three, and the last of the stage-2 sequence. Same chart as
1.0.0+upjava8 and 1.0.0+upjava17, published for the third Java line, so
main comes to rest on the line of the only server that is actually
running.

minecraft.version moves with it (1.20.1 -> 1.21.1): a Minecraft version
only runs on a matching JVM, and 1.20.5+ needs Java 21. This default now
equals what ftb-skies-2 sets in its own values file, so the install-test
boots the production version rather than an arbitrary Java-21-capable one.

The CI values deliberately do not override minecraft.version, so this
default is what the install-test exercises.

minecraft-server/Chart.yaml

@@ -8,8 +8,8 @@ type: application
 # The Java line of this chart version is chosen through appVersion, which is
 # the image tag: the chart is published once per line (java8 / java17 / java21)
 # and minecraft.version in values.yaml must match the line published here.
-version: 1.0.0+upjava17
+version: 1.0.0+upjava21
-appVersion: "java17"
+appVersion: "java21"
 
 maintainers:
   - name: jklein

minecraft-server/values.yaml

@@ -184,8 +184,8 @@ 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 "java17".
+  # 1.20.5+ need Java 21. This chart publishes appVersion "java21".
-  version: "1.20.1"
+  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