Change PausedProblem rateLimited to userActionRequired

[pgporada]

When initially designing account pausing/unpausing, I believe we overlooked a more applicable error that could be returned to the client. Currently we return ProblemType("rateLimited"), but instead I think we could implement and return ProblemType("userActionRequired"). RFC 8555 states,

| userActionRequired      | Visit the "instance" URL and take       |
|                         | actions specified there                 |

Now, the only example in the RFC about userActionRequired is for Changes of Terms of Service which we have done in the past, but not using this error type either.

If it's decided that boulder wants to use this, this comment should also be updated.

Part of #7406