Telegram Integration: escape markdown characters

What does this MR do and why?

As a part of the #431467 (closed), we have to fix Telegram's integration in case we're having broken markdown syntax.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
Nothing

How to set up and validate locally

1. Set up a Telegram integration
2. Update your username to include some markdown special character, e.g. _Administrator
3. Try to test Telegram's integration with Test Settings button
4. Verify there's no message coming to the channel
5. Checkout the branch
6. Try to test the integration again
7. Verify that everything works as expected! 🎉

Related to #431467 (closed)