[Notifier] Improve Telegrams markdown escaping

[codedge]

Q	A
Branch?	6.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	Fix #51330
License	MIT

This PR reverts/improves the escaping of Telegrams MarkdownV2 characters. It fixes the currently broken escaping of every single Markdown char while not taking into account that they come in pairs.

[carsonbot]

Hey!

I see that this is your first PR. That is great! Welcome!

Symfony has a contribution guide which I suggest you to read.

In short:

• Always add tests
• Keep backward compatibility (see https://symfony.com/bc).
• Bug fixes must be submitted against the lowest maintained branch where they apply (see https://symfony.com/releases)
• Features and deprecations must be submitted against the 7.2 branch.

Review the GitHub status checks of your pull request and try to solve the reported issues. If some tests are failing, try to see if they are failing because of this change.

When two Symfony core team members approve this change, it will be merged and you will become an official Symfony contributor!
If this PR is merged in a lower version branch, it will be merged up to all maintained branches within a few days.

I am going to sit back now and wait for the reviews.

Cheers!

Carsonbot

[nicolas-grekas]

Thank you @codedge.