Pin minecraft-server to dated image tags and fix the whitelist switch (2.0.0+up2026.8.2.java8)

Stage 3 of the Fleet migration. Two changes, both breaking, hence the major
bump from 1.0.0 to 2.0.0.

Dated image tags: appVersion moves from the rolling line tag "java8" to the
immutable upstream release tag "2026.8.2-java8". The rolling tags are rebuilt
continuously, so a pod restart could pull a different image than the previous
start without that being recorded anywhere. 2026.8.2 is the newest upstream
release (published 2026-08-23) and exists for all three Java lines.

Whitelist semantics: the chart now wires ENABLE_WHITELIST (and ENFORCE_WHITELIST)
in both positions, so minecraft.whitelist.enabled: false actually switches the
whitelist off. Previously the chart only emitted whitelist variables while
enabled was true and had no way to express "off" - and because server.properties
lives on the PVC, a release that had once written white-list=true kept it
forever. players stays guarded by enabled, since a non-empty WHITELIST enables
the whitelist in the image regardless of ENABLE_WHITELIST.

The default flips from enabled: true to enabled: false. Under the corrected
semantics the old default (enabled with an empty player list) would mean a
server nobody can join; false matches both the image and vanilla Minecraft.

minecraft-server/Chart.yaml

@@ -8,8 +8,13 @@ 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+upjava21
-appVersion: "java21"
+# Since chart 2.0.0 the tag is a DATED upstream release ("<release>-java<N>")
+# instead of the rolling line tag ("java<N>"): the rolling tags are rebuilt
+# 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.java8
+appVersion: "2026.8.2-java8"
 
 maintainers:
   - name: jklein