Increase rc_invites.per_issuer for Complement

[MadLittleMods]

Increase rc_invites.per_issuer (invite rate limit) for Complement

It's possible to run into SynapseError: 429 - Too Many Requests (rc_invites.per_issuer)

rc_invites.per_issuer was originally introduced in matrix-org/synapse#13125

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

Config documentation for reference: https://github.com/element-hq/synapse/blob/aab3672037ee9ea71c54c89c86d9c0cc35ae184f/docs/usage/configuration/config_documentation.md#rc_invites

Looks like we just forgot to define this one so it's set really low with the default.

[MadLittleMods]

Thanks for the review @anoadragon453 🦒