Update label of select branch dropdown on chat integrations edit page

What does this MR do and why?

This MR updates the label of the select for branches_to_be_notified dropdown from Branches to be notified to Branches for which notifications are to be sent. This is applied for Google chat as well as other chat integrations (Discord, MS Teams...)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Visit http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/services/hangouts_chat/edit

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #328681 (closed)