Move project to Java 21 (#17)

* Update build scripts and docs
* Adjust module to remove `requires transitive` of automatic module

.github/website.sh

@@ -21,4 +21,9 @@ git commit --message "Update joda-collect from CI: $GITHUB_ACTION"
 echo "## push..."
 git push origin main
 
+echo "## tidy..."
+cd ..
+git push --delete origin website || true
+git push --delete origin website2x || true
+
 echo "## done"

.github/workflows/build.yml

@@ -19,9 +19,6 @@ jobs:
     permissions:
       security-events: write  # for github/codeql-action
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        java: [8, 11]
     steps:
     - name: Checkout
       uses: actions/checkout@v4
@@ -31,7 +28,7 @@ jobs:
     - name: Set up JDK
       uses: actions/setup-java@v4
       with:
-        java-version: ${{ matrix.java }}
+        java-version: 21
         distribution: 'temurin'
         cache: 'maven'
 
@@ -55,7 +52,7 @@ jobs:
       uses: github/codeql-action/analyze@v3
 
     - name: Website
-      if: matrix.java == '11' && github.event_name == 'push' && (startsWith(github.ref, 'refs/tags/website') || startsWith(github.ref, 'refs/tags/v'))
+      if: github.event_name == 'push' && (startsWith(github.ref, 'refs/tags/website') || startsWith(github.ref, 'refs/tags/v'))
       env:
         GITHUB_TOKEN: ${{ secrets.PERSONAL_TOKEN_GH }}
       run: |

README.md

@@ -27,9 +27,14 @@ Various documentation is available:
 
 
 ### Releases
-[Release 1.0.1](https://www.joda.org/joda-collect/download.html) is the current latest release.
-This release is considered stable and worthy of the 1.x tag.
-It depends on Java SE 8 or later and requires [Google-Guava](https://github.com/google/guava).
+The 2.x branch is compatible with Java SE 21 or later.
+
+The 1.x branch is compatible with Java SE 8 or later.
+
+v2.x releases are compatible with v1.x releases - except for the Java SE version and `module-info.class` file.
+Guava is a required module, but it cannot be declared as 'transitive' because it is an automatic module.
+
+Joda-Collect depends on [Google-Guava](https://github.com/google/guava).
 
 Available in the [Maven Central repository](https://search.maven.org/search?q=g:org.joda%20AND%20a:joda-collect&core=gav)
 
@@ -59,7 +64,9 @@ Tidelift will coordinate the fix and disclosure.
 
 ### Release process
 
-* Update version (README.md, index.md, changes.xml)
+* Update version (index.md, changes.xml)
 * Commit and push
+* Switch to Java 21
 * `mvn clean release:clean release:prepare release:perform`
+* `git fetch`
 * Website will be built and released by GitHub Actions

RELEASE-NOTES.txt

@@ -3,7 +3,7 @@ Joda-Collect
 ================================================
 Joda-Collect provides collections that are not present in the JDK or [Google Guava](https://github.com/google/guava).
 
-The release runs on JDK 8 or later.
+The release runs on JDK 21 or later.
 
 See https://www.joda.org/joda-collect/changes-report.html for changes