refactor: apply modernize linter suggestions

[k1LoW]

Use slices.Contains/ContainsFunc, maps.Copy, strings.SplitSeq, range-over-int, and WaitGroup.Go to satisfy golangci-lint modernize.

[github-actions]

Code Metrics Report

	main (44a6df1)	#113 (c923b42)	+/-
Coverage	52.8%	52.8%	+0.0%
Code to Test Ratio	1:0.5	1:0.5	-0.1
Test Execution Time	28s	28s	0s

Details

  |                     | main (44a6df1) | #113 (c923b42) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          52.8% |          52.8% | +0.0% |
  |   Files             |             37 |             37 |     0 |
  |   Lines             |           2941 |           2935 |    -6 |
- |   Covered           |           1555 |           1552 |    -3 |
- | Code to Test Ratio  |          1:0.5 |          1:0.5 |  -0.1 |
  |   Code              |           4570 |           4572 |    +2 |
  |   Test              |           2619 |           2619 |     0 |
  | Test Execution Time |            28s |            28s |    0s |

Code coverage of files in pull request scope (26.1% → 26.0%)

Files	Coverage	+/-	Status
cmd/root.go	22.6%	-0.3%	modified
internal/server/group.go	100.0%	0.0%	modified

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Reported by octocov