minor #15816 [Mailer] Update mailer.rst (gnito-org)

This PR was submitted for the 5.3 branch but it was merged into the 5.4 branch instead.

Discussion
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[Mailer] Update mailer.rst

Add Sendgrid to the list of transports that support tags and metadata.

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Commits
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fc4f386 Update mailer.rst

Author: javiereguiluz

mailer.rst

@@ -1331,6 +1331,7 @@ The following transports currently support tags and metadata:
 * MailChimp
 * Mailgun
 * Postmark
+* Sendgrid
 * Sendinblue
 
 The following transports only support tags: