Change GitHub Workflows to use "temurin" Java.

(Docs for https://github.com/actions/setup-java were changed a few days ago to warn that "adopt" Java won't be updated anymore)

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
         uses: actions/setup-java@v2.1.0
         with:
           java-version: 8
-          distribution: 'adopt'
+          distribution: 'temurin'
 
       - name: Cache local Maven repository
         uses: actions/cache@v2

.github/workflows/release.yml

@@ -31,7 +31,7 @@ jobs:
         uses: actions/setup-java@v2.1.0
         with:
           java-version: 8
-          distribution: 'adopt'
+          distribution: 'temurin'
 
       # If this run stores a new cache then the next run will not be able to use it because this is a tag checkout :-(
       # But a cache already created on the default branch (by the CI workflow) will be loaded here.