Deprecate email credentials from environment variables.

[jmcarp]

Email backends fetch credentials from environment variables, but other
credentials are typically stored in connections. This patch deprecates
email credentials from environment variables and checks connections
first. We can drop the environment variable fallback in a future
release.

Based on conversation in #13463. I would like to standardize credential handling in email backends, then add an SES backend separately. I'm marking this as a draft for feedback, and I'll write tests once we agree on an approach. cc @mik-laj

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The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

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The Workflow run is cancelling this PR. It has some failed jobs matching ^Pylint$,^Static checks,^Build docs$,^Spell check docs$,^Backport packages$,^Provider packages,^Checks: Helm tests$,^Test OpenAPI*.

[jmcarp]

OK, ready for review!

[potiuk]

Nice