harden entrygroups

[arnested]

• Fix golangci-lint warning
• Avoid deprecated types
• Use longer variable name
• Avoid keeping an entrygroup locked

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 30.81%. Comparing base (720986d) to head (8d871eb).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
entry_groups.go	0.00%	7 Missing ⚠️
container.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
- Coverage   30.98%   30.81%   -0.18%     
==========================================
  Files           7        7              
  Lines         355      357       +2     
==========================================
  Hits          110      110              
- Misses        243      245       +2     
  Partials        2        2              

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