fix: index for MAU

[sattvikc]

Summary of change

Adds index for MAU

Related issues

• Very high response time for setup licence key api with existing data #994

Test Plan

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your
changes work. Bonus points for screenshots and videos!)

Documentation changes

(If relevant, please create a PR in our docs repo, or create a checklist here
highlighting the necessary changes)

Checklist for important updates

• Changelog has been updated
  • If there are any db schema changes, mention those changes clearly
• coreDriverInterfaceSupported.json file has been updated (if needed)
• pluginInterfaceSupported.json file has been updated (if needed)
• Changes to the version if needed
  • In build.gradle
• If added a new paid feature, edit the getPaidFeatureStats function in FeatureFlag.java file
• Had installed and ran the pre-commit hook
• If there are new dependencies that have been added in build.gradle, please make sure to add them
  in implementationDependencies.json.
• Update function getValidFields in io/supertokens/config/CoreConfig.java if new aliases were added for any core
  config (similar to the access_token_signing_key_update_interval config alias).
• Issue this PR against the latest non released version branch.
  • To know which one it is, run find the latest released tag (git tag) in the format vX.Y.Z, and then find the
    latest branch (git branch --all) whose X.Y is greater than the latest released tag.
  • If no such branch exists, then create one from the latest released branch.
• If added a foreign key constraint on app_id_to_user_id table, make sure to delete from this table when deleting
  the user as well if deleteUserIdMappingToo is false.

Remaining TODOs for this PR

• Item1
• Item2