[Mailer][Notifier] Update Sendinblue / Brevo API host

[vicla31]

Q	A
Branch?	5.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	Fix #52264
License	MIT

Sendinblue rebrands to Brevo, and it simultaneously upgrade smtp and API servers.
There are 2 distincts API currently : api.sendinblue.com (deprecated but still running) and api.brevo.com (the one to use)
Unlike SMTP host, API host is not changed on LTS 5.4 and are still set to api.sendinblue.com.

[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 6.4 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 @vicla31.