Improve index file invalidation message in Maven plugin

Do not mention plugin fingerprint because it is an implementation detail and
can be confusing to users. Explain why index file invalidation could happen.

Author: lutovich

plugin-maven/src/main/java/com/diffplug/spotless/maven/incremental/FileIndex.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 DiffPlug
+ * Copyright 2021-2023 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -76,7 +76,7 @@ static FileIndex read(FileIndexConfig config, Log log) {
 			PluginFingerprint computedFingerprint = config.getPluginFingerprint();
 			PluginFingerprint storedFingerprint = PluginFingerprint.from(firstLine);
 			if (!computedFingerprint.equals(storedFingerprint)) {
-				log.info("Fingerprint mismatch in the index file. Fallback to an empty index");
+				log.info("Index file corresponds to a different configuration of the plugin. Either the plugin version or its configuration has changed. Fallback to an empty index");
 				return emptyIndexFallback(config);
 			} else {
 				Content content = readIndexContent(reader, config.getProjectDir(), log);

plugin-maven/src/test/java/com/diffplug/spotless/maven/incremental/FileIndexTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 DiffPlug
+ * Copyright 2021-2023 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -60,7 +60,7 @@ void readFallsBackToEmptyIndexOnFingerprintMismatch() throws Exception {
 		FileIndex index = FileIndex.read(config, log);
 
 		assertThat(index.size()).isZero();
-		verify(log).info("Fingerprint mismatch in the index file. Fallback to an empty index");
+		verify(log).info("Index file corresponds to a different configuration of the plugin. Either the plugin version or its configuration has changed. Fallback to an empty index");
 	}
 
 	@Test