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