fix: correct email headers (mime version and content-type)

[stebenz]

As noticed by a customer (relates to #7880), the Mime-Version and Content-Type were not properly formatted, which depending on the smtp relay led to bad spam scoring.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 63.00%. Comparing base (6c0e7c4) to head (f74c4e9).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/notification/messages/email.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7886      +/-   ##
==========================================
- Coverage   63.00%   63.00%   -0.01%     
==========================================
  Files        1333     1333              
  Lines      110078   110078              
==========================================
- Hits        69354    69350       -4     
- Misses      36775    36779       +4     
  Partials     3949     3949              

Flag	Coverage Δ
core-integration-tests-postgres	63.00% <0.00%> (-0.01%)	⬇️

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[github-actions]

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