chore(waitgroup): update usage to Go 1.25

[Elia-Renzoni]

This PR updates the usage of WaitGroups to align with Go 1.25.

Changes

• Update WaitGroup usage to the Go 1.25 API.
• Remove closures that previously captured loop/index variables.
• Introduce helper variables to avoid index-out-of-bound errors that appeared after removing those closures.

Notes

• The helper variables were added because the update removed closures that previously enclosed loop/index variables; without these helpers the code produced index-out-of-bound errors.
• If the current helper variable names are unclear or not descriptive enough, I'm happy to rename them based on your suggestions.
• Some parts of the updated code may still be optimized further; I welcome suggestions and feedback on possible improvements.

Related issue: #870

Thanks for reviewing — any feedback is appreciated.

[coveralls]

Pull Request Test Coverage Report for Build 19391845735

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 89 of 96 (92.71%) changed or added relevant lines in 15 files are covered.
• 66 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 76.2%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/backends/clickhouse/model/unmarshal.go	0	1	0.0%
pkg/timeseries/epoch/epoch.go	0	1	0.0%
pkg/proxy/engines/proxy_request.go	7	9	77.78%
pkg/timeseries/dataset/series_list.go	4	7	57.14%

Files with Coverage Reduction	New Missed Lines	%
pkg/proxy/engines/deltaproxycache.go	2	83.52%
pkg/cache/index/client.go	64	72.18%

Totals
Change from base Build 17437024522:	0.01%
Covered Lines:	15285
Relevant Lines:	20059

---

💛 - Coveralls

[Elia-Renzoni]

@jranson I was wondering if it might be possible to merge it, so I can work on some new changes without running into conflicts.
Please let me know if there’s anything else I should adjust.
Thanks a lot 🙏

[jranson]

@Elia-Renzoni thanks for this, and sorry I needed more time to review it. As of go 1.22, for loop variables are properly assigned per-iteration, so they should be concurrency-safe when passed into goroutines. See notes below for when you can do that without needing any loop helper vars. The index-out-of-bounds errors are specific to variables that are not controlled by / declared in the for clause, so those non-clause helpers were OK to keep but must be declared before/outside of the wg.Go() to avoid data race conditions.