minor #11538 [Mime] Use constants instead of numbers to set email priority (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #11538).

Discussion
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[Mime] Use constants instead of numbers to set email priority

Today I learned about these constants. I think it makes code more readable.

Commits
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632d8e1 [Mime] Use constants instead of numbers to set email priority

Author: OskarStark

components/mime.rst

@@ -50,7 +50,7 @@ methods to compose the entire email message::
         ->cc('bar@example.com')
         ->bcc('baz@example.com')
         ->replyTo('fabien@symfony.com')
-        ->priority(1)
+        ->priority(Email::PRIORITY_HIGH)
         ->subject('Important Notification')
         ->text('Lorem ipsum...')
         ->html('<h1>Lorem ipsum</h1> <p>...</p>')