Rollup merge of #152182 - apiraino:update-compiler-stable-backport-zulip-msg, r=jieyouxu

update compiler stable backport zulip msg

I'd like to update the message when stable backports (for t-compiler) Zulip topics are opened.

Stable backports mention the channel i.e. also people who might not have context, example:
[#t-compiler/backports > #150590: stable-nominated @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/474880-t-compiler.2Fbackports/topic/.23150590.3A.20stable-nominated/near/569989784)

Beta backports mention author+reviewer (which fits better), example:
[#t-compiler/backports > #151896: beta-nominated @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/474880-t-compiler.2Fbackports/topic/.23151896.3A.20beta-nominated/near/571171604)

This patch makes the `stable` backport opening message just like the `beta` backport one.

Thanks

Author: JonathanBrouwer