Update some build tooling

Mostly now use JDK 25 to build, to allow us to use markdown docstrings
in the future — this makes the javadoc generator *much* simpler.

.github/workflows/main-ci.yml

@@ -14,7 +14,7 @@ jobs:
     - name: 📥 Set up Java
       uses: actions/setup-java@v4
       with:
-        java-version: 21
+        java-version: 25
         distribution: 'temurin'
 
     - name: 📥 Setup Gradle
@@ -87,7 +87,7 @@ jobs:
     - name: 📥 Set up Java
       uses: actions/setup-java@v4
       with:
-        java-version: 21
+        java-version: 25
         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: 21
+        java-version: 25
         distribution: 'temurin'
 
     - name: 📥 Setup Gradle