chore: don't require connection_uri in SMTP

[jonas-jonas]

Related issue(s)

(Actually) Closes #3559 and closes #3693

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.02%. Comparing base (2cdfc70) to head (7f4dc72).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3861      +/-   ##
==========================================
- Coverage   78.04%   78.02%   -0.03%     
==========================================
  Files         360      360              
  Lines       25233    25203      -30     
==========================================
- Hits        19694    19665      -29     
- Misses       4033     4034       +1     
+ Partials     1506     1504       -2     

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