[hotfix] omit recipient number from rate limit

context

• while we are ironing out load issues, we send maintainers notifications of rate-limit errors
• these messages needlessly include the recipient phone number which (1) provides no useful info, (2) leaks user data! (although they always disappear in a day, so the leakage is mitigated)

changes

• to better protect our users' privacy just send the channel number in alerts, not the recipient number