chore: modernize code

[jkroepke]

What this PR does / why we need it

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

• fixes #

Special notes for your reviewer

Particularly user-facing changes

Checklist

Complete these before marking the PR as ready to review:

• DCO signed
• The PR title has a summary of the changes
• The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 81.40351% with 106 lines in your changes missing coverage. Please review.

Project coverage is 82.82%. Comparing base (1a503b5) to head (88ba3ec).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/oauth2/handler.go	63.63%	24 Missing and 8 partials ⚠️
internal/oauth2/utils.go	62.16%	10 Missing and 4 partials ⚠️
internal/oauth2/refresh.go	73.80%	6 Missing and 5 partials ⚠️
internal/openvpn/passthrough.go	75.67%	6 Missing and 3 partials ⚠️
cmd/daemon/root.go	85.18%	7 Missing and 1 partial ⚠️
internal/oauth2/provider.go	91.30%	5 Missing and 3 partials ⚠️
internal/openvpn/main.go	85.71%	4 Missing and 2 partials ⚠️
internal/openvpn/handler.go	88.88%	2 Missing and 2 partials ⚠️
internal/httphandler/handler.go	88.00%	2 Missing and 1 partial ⚠️
internal/oauth2/providers/generic/check.go	88.88%	2 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
- Coverage   82.92%   82.82%   -0.10%     
==========================================
  Files          51       50       -1     
  Lines        3162     2859     -303     
==========================================
- Hits         2622     2368     -254     
+ Misses        412      376      -36     
+ Partials      128      115      -13     

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