feat(nimbus): Add prefix channel name in the slack DM message

[yashikakhurana]

Because

• We are sending DMs to the user if they are not in the channel about the slack message, but we are not including the channel name to promote them to join the channel

This commit

• Adds the channel name in the message

Fixes #14215

[jaredlockhart]

Aren't there tests?

[yashikakhurana]

Aren't there tests?
yup there are, my bad, skipped to commit that file

changes done

[jaredlockhart]

Very nice ty @yashikakhurana 🙏