Update GE injection script for Develocity rename

- Rename all env vars expected by init-script: `s/GRADLE_ENTERPRISE/DEVELOCITY`
- Update README for changes
- Use `Develocity` consistently in tests

dist/post/index.js

@@ -136189,7 +136189,7 @@ class GradleStateCache {
             'gradle-build-action.build-result-capture-service.plugin.groovy',
             'gradle-build-action.github-dependency-graph.init.gradle',
             'gradle-build-action.github-dependency-graph-gradle-plugin-apply.groovy',
-            'gradle-build-action.inject-gradle-enterprise.init.gradle'
+            'gradle-build-action.inject-develocity.init.gradle'
         ];
         for (const initScriptFilename of initScriptFilenames) {
             const initScriptContent = this.readResourceFileAsString('init-scripts', initScriptFilename);