minor #39465 [Mailer] [Sendgrid] Use $scheme variable to stay consistent (OskarStark)

minor #39465 [Mailer] [Sendgrid] Use $scheme variable to stay consistent (OskarStark)

This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] [Sendgrid] Use $scheme variable to stay consistent

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | ---
| License       | MIT
| Doc PR        | ---

Its used like this in all other mailer bridges

Commits
-------

1a40d7c75a [Mailer] [Sendgrid] Use $scheme variable to stay consistent