[Stable plugin api] Clean up comments and variable names in gradle (#93501)

in example plugins a pluginApiVersion is more appropriate than elasticsearchVersion as it better implies that it can be different then the version of ES cluster

Author: pgomulka

build-tools/src/main/resources/plugin-descriptor.properties

@@ -1,5 +1,5 @@
 # Elasticsearch plugin descriptor file
-# This file must exist as 'plugin-descriptor.properties' inside a plugin.
+# This file must exist as 'plugin-descriptor.properties' or 'stable-plugin-descriptor.properties inside a plugin.
 #
 ### example plugin for "foo"
 #
@@ -31,7 +31,9 @@ name=${name}
 # separated by "."'s and may have leading zeros
 java.version=${javaVersion}
 #
-# 'elasticsearch.version': version of elasticsearch compiled against
+# 'elasticsearch.version': version of elasticsearch compiled against.
+# Plugins implementing plugin-api.jar this version only has to match a major version of the ES server
+# For all other plugins it has to be the same as ES server version
 elasticsearch.version=${elasticsearchVersion}
 ### optional elements for plugins:
 <% if (classname) { %>

plugins/examples/settings.gradle

@@ -22,6 +22,8 @@ gradle.rootProject {
       def props = new Properties()
       props.load(is)
       elasticsearchVersion = "${props.get('elasticsearch')}-SNAPSHOT"
+      // for example plugins the pluginApiVersion is the same as ES version, but for real plugins it only has to match a major
+      pluginApiVersion = "${props.get('elasticsearch')}-SNAPSHOT"
       log4jVersion = props.get('log4j')
       luceneVersion = props.get('lucene')
     }

plugins/examples/stable-analysis/build.gradle

@@ -10,13 +10,13 @@ esplugin {
 dependencies {
 
   //TODO transitive dependency off and plugin-api dependency?
-  compileOnly "org.elasticsearch.plugin:elasticsearch-plugin-api:${elasticsearchVersion}"
-  compileOnly "org.elasticsearch.plugin:elasticsearch-plugin-analysis-api:${elasticsearchVersion}"
+  compileOnly "org.elasticsearch.plugin:elasticsearch-plugin-api:${pluginApiVersion}"
+  compileOnly "org.elasticsearch.plugin:elasticsearch-plugin-analysis-api:${pluginApiVersion}"
   compileOnly "org.apache.lucene:lucene-analysis-common:${luceneVersion}"
 
   //TODO for testing this also have to be declared
-  testImplementation "org.elasticsearch.plugin:elasticsearch-plugin-api:${elasticsearchVersion}"
-  testImplementation "org.elasticsearch.plugin:elasticsearch-plugin-analysis-api:${elasticsearchVersion}"
+  testImplementation "org.elasticsearch.plugin:elasticsearch-plugin-api:${pluginApiVersion}"
+  testImplementation "org.elasticsearch.plugin:elasticsearch-plugin-analysis-api:${pluginApiVersion}"
   testImplementation "org.apache.lucene:lucene-analysis-common:${luceneVersion}"
 
   testImplementation ('junit:junit:4.13.2'){