Breaking: Rename the environment variable for self-signed email certificates

[stumpylog]

Proposed change

I should have named this differently in the first place. It gets read during the env-from-file, which could be confusing, as it will read the file and set a variable with the _FILE trimmed. It's not needed and a user might wonder what's happening.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain):

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[shamoon]

This will be a breaking change, yea?

[stumpylog]

Yes. Since there's already some for v2, this seems like the best time to do so.

[codecov]

Codecov Report

Merging #4346 (31b7250) into dev (e52ae28) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #4346   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files         359      359           
  Lines       13696    13696           
  Branches     1092     1092           
=======================================
  Hits        13119    13119           
  Misses        573      573           
  Partials        4        4           

Flag	Coverage Δ
backend	94.49% <100.00%> (ø)
frontend	97.30% <ø> (ø)

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Files	Coverage Δ
src/paperless/settings.py	89.41% <100.00%> (ø)

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[shamoon]

Cool, yea agree, as good a time as any

[github-actions]

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