R40001XWIKI7540DataMigration is skipped unintentionally

The skipping of the migration done in XWIKI-8036 was not implemented in the right place. It seems that the shouldExecute() method is not able to properly read the database (to read pages and objects) because it is executed before any of the migrations are executed.

To fix this, the code that checks the annotation and comments class needs to be executed inside the migration itself. This makes sure that all the previous migrations are executed and that the database is in a readable state, at least for the current code.