text: Title: Default URL param value for Gravatar URL have been deprecated (`mm` -> `mp`)
  Body: ### Issue Summary
  
  We currently pass in `mm` to the `d` (default) param, this is used to determine what avatar will show if there's no matching avatar. However, the latest documentation advises that this should be `mp` (mystery person) instead.
  
  https://github.com/wagtail/wagtail/blob/c2676af857a41440e05e03038d85a540dcca3ce2/wagtail/users/utils.py#L28-L29
  
  https://github.com/wagtail/wagtail/blob/c2676af857a41440e05e03038d85a540dcca3ce2/wagtail/users/utils.py#L45
  
  https://docs.gravatar.com/api/avatars/images/#default-image
  
  
  ### Describe the solution you'd like
  
  Update the param value from `mm` to `mp` and ensure any unit tests are updated.
  
  This way, if the support for this legacy value gets dropped, it will not be a breaking change for Wagtail users.
  
  ### Describe alternatives you've considered
  
  It might be nice to have a better approach to this by allowing the param to be passed into the function / overridden somehow. Best to discuss that in a different issue though - see https://github.com/wagtail/wagtail/issues/12659
  
  ### Additional context
  
  Two PRs have attempted this (and other changes), see the feedback and the PRs for reference.
  
  - #11077
  - #11800
  
  ### Working on this
  
  - Anyone can contribute to this, be sure you understand how to reproduce the avatar scenario.
  - It might be good to tackle this small change before tackling the other related issues.
  - View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.
  