fix(php-yoshi): set "version" if it's in composer.json (#2192)

Co-authored-by: Jeff Ching <chingor@google.com>

src/strategies/php-yoshi.ts

@@ -114,6 +114,7 @@ export class PHPYoshi extends BaseStrategy {
           splitCommits[directory]
         );
         versionsMap.set(composer.name, newVersion);
+        versionsMap.set('version', newVersion);
         const partialReleaseNotes = await this.changelogNotes.buildNotes(
           splitCommits[directory],
           {