Rollup merge of #147527 - apiraino:update-beta-nom-zulip-msg, r=Kobzol

Update t-compiler beta nomination Zulip msg

Sister patch of [triagebot#2191](rust-lang/triagebot#2191)
Follow-up to rust-lang/rust#147263

The triagebot now triggers a different message when a PR is nominated for backport, making it look like more a suggestion to evaluate for the author/reviewers than a mandatory decision for the team to take.

The wording (as per [triagebot#2191](rust-lang/triagebot#2191)) is open to suggestions.

Thanks

Author: matthiaskrgr