ci: remove Drone CI configuration

All build systems now GitHub Actions workflows, so we can remove the
alternative configuration and only keep a single pipeline definition.

.drone.yml

@@ -1,31 +0,0 @@
-kind: pipeline
-type: docker
-name: java11
-
-steps:
-  - name: test
-    image: eclipse-temurin:11-jdk
-    commands:
-      - ./mvnw -B clean test
-
----
-kind: pipeline
-type: docker
-name: java17
-
-steps:
-  - name: test
-    image: eclipse-temurin:17-jdk
-    commands:
-      - ./mvnw -B clean test
-
----
-kind: pipeline
-type: docker
-name: java21
-
-steps:
-  - name: test
-    image: eclipse-temurin:21-jdk
-    commands:
-      - ./mvnw -B clean test

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Test
         run: ./mvnw -B -P coverage clean verify
       - name: Analysis
-        if: matrix.analysis
+        if: matrix.analysis && env.SONAR_TOKEN != ''
         run: >
           ./mvnw -B sonar:sonar
           -Dsonar.host.url=https://sonarcloud.io