Update CI to use Java 21

.github/workflows/main-ci.yml

@@ -14,7 +14,7 @@ jobs:
     - name: 📥 Set up Java
       uses: actions/setup-java@v4
       with:
-        java-version: 17
+        java-version: 21
         distribution: 'temurin'
 
     - name: 📥 Setup Gradle
@@ -87,7 +87,7 @@ jobs:
     - name: 📥 Set up Java
       uses: actions/setup-java@v4
       with:
-        java-version: 17
+        java-version: 21
         distribution: 'temurin'
 
     - name: 📥 Setup Gradle

.github/workflows/make-doc.yml

@@ -17,7 +17,7 @@ jobs:
     - name: 📥 Set up Java
       uses: actions/setup-java@v4
       with:
-        java-version: 17
+        java-version: 21
         distribution: 'temurin'
 
     - name: 📥 Setup Gradle

CONTRIBUTING.md

@@ -28,7 +28,7 @@ Translations are managed through [CrowdIn], an online interface for managing lan
 In order to develop CC: Tweaked, you'll need to download the source code and then run it.
 
  - Make sure you've got the following software installed:
-   - Java Development Kit 17 (JDK). This can be downloaded from [Adoptium].
+   - Java Development Kit 21 (JDK). This can be downloaded from [Adoptium].
    - [Git](https://git-scm.com/).
    - [NodeJS 20 or later][node].
 
@@ -101,7 +101,7 @@ about how you can build on that until you've covered everything!
 
 [new-issue]: https://github.com/cc-tweaked/CC-Tweaked/issues/new/choose "Create a new issue"
 [community]: README.md#community "Get in touch with the community."
-[Adoptium]: https://adoptium.net/temurin/releases?version=17 "Download OpenJDK 17"
+[Adoptium]: https://adoptium.net/temurin/releases?version=21 "Download OpenJDK 21"
 [illuaminate]: https://github.com/SquidDev/illuaminate/ "Illuaminate on GitHub"
 [docs]: https://tweaked.cc/ "CC: Tweaked documentation"
 [ldoc]: http://stevedonovan.github.io/ldoc/ "ldoc, a Lua documentation generator."

buildSrc/src/main/kotlin/cc-tweaked.java-convention.gradle.kts

@@ -161,7 +161,7 @@ tasks.javadoc {
     options {
         val stdOptions = this as StandardJavadocDocletOptions
         stdOptions.addBooleanOption("Xdoclint:all,-missing", true)
-        stdOptions.links("https://docs.oracle.com/en/java/javase/17/docs/api/")
+        stdOptions.links("https://docs.oracle.com/en/java/javase/21/docs/api/")
     }
 }