Miscellaneous fixes for Meilisearch

It appears Meilisearch has been broken for quite some time, at least for new installations. This should take care of that.

• First, the Mix task uses POST requests for creating indexes whis has been deprecated and removed for at least two years.
• Then initial indexing might still crash if one happens to have malformed Note objects with nil date federated. I think those can be safely ignored when attempted to be indexed.
• And finally, instructions for OTP releases list incorrect command for Mix tasks that just throws an error.

Checklist

• Adding a changelog: In the changelog.d directory, create a file named <code>.<type>.

  <code> can be anything, but we recommend using a more or less unique identifier to avoid collisions, such as the branch name.

  <type> can be add, change, remove, fix, security or skip. skip is only used if there is no user-visible change in the MR (for example, only editing comments in the code). Otherwise, choose a type that corresponds to your change.

  In the file, write the changelog entry. For example, if an MR adds group functionality, we can create a file named group.add and write Add group functionality in it.

  If one changelog entry is not enough, you may add more. But that might mean you can split it into two MRs. Only use more than one changelog entry if you really need to (for example, when one change in the code fix two different bugs, or when refactoring).