Designate java21 as the latest/default image (#2757)

.github/ISSUE_TEMPLATE/bug.yml

@@ -11,9 +11,9 @@ body:
  
  Please double check some things first:
  1. Do you just have a question about something? If so, asking in the [Q&A Discussions](https://github.com/itzg/docker-minecraft-server/discussions/categories/q-a) or asking on [the Discord server](https://discord.gg/DXfKpjB) would be best.
- 2. Did you re-pull the newest image using `docker pull`? 
- 3. Is this bug happening after the `[init]` prefixed logs and after the log that says "Starting the Minecraft server"? If so, please report the bug with Mojang or the respective server provider.
- 4. Are you seeing a performance problem? If so, that is typically outside the scope of the image setup mechanims. Ask a question as above or contact the respective server provider.
+ 2. Did you **re-pull the newest image** and confirmed the issue after that? Run `docker pull <image>` or if using a compose file, it's as easy as running `docker compose pull`. If using Kubernetes, add `imagePullPolicy: Always` to the container. 
+ 4. Is this bug happening after the `[init]` prefixed logs and after the log that says "Starting the Minecraft server"? If so, please report the bug with Mojang or the respective server provider.
+ 5. Are you seeing a performance problem? If so, that is typically outside the scope of this image. Ask a question as above or contact the respective server provider.
  - type: textarea
  id: problem
  attributes:

.github/workflows/build-multiarch.yml

@@ -107,8 +107,8 @@ jobs:
  env:
  IMAGE_TO_TEST: ${{ github.repository_owner }}/minecraft-server:test-${{ matrix.variant }}-${{ github.run_id }}
  HAS_IMAGE_REPO_ACCESS: ${{ secrets.DOCKER_USER != '' && secrets.DOCKER_PASSWORD != '' }}
- MAIN_VARIANT: java17
- runs-on: ubuntu-20.04
+ MAIN_VARIANT: java21
+ runs-on: ubuntu-22.04
  steps:
  - name: Checkout
  uses: actions/checkout@v4.1.2

.github/workflows/stale-check.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
  check:
- runs-on: ubuntu-20.04
+ runs-on: ubuntu-22.04
  permissions:
  issues: write
  pull-requests: write

.github/workflows/verify-pr.yml

@@ -42,7 +42,7 @@ jobs:
  mcVersion: 1.12.2
  env:
  IMAGE_TO_TEST: ${{ github.repository_owner }}/minecraft-server:test-${{ matrix.variant }}-${{ github.run_id }}
- runs-on: ubuntu-20.04
+ runs-on: ubuntu-22.04
  steps:
  - name: Checkout
  uses: actions/checkout@v4.1.2