[7.15](backport #28725) Change proxy_url from url.URL to string

[mergify]

This is an automatic backport of pull request #28725 done by Mergify.
Cherry-pick of f025c96 has failed:

On branch mergify/bp/7.15/pr-28725
Your branch is up to date with 'origin/7.15'.

You are currently cherry-picking commit f025c96219.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
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Changes to be committed:
	modified:   CHANGELOG.next.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/libbeat/common/aws/credentials.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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• Start Time: 2021-11-03T22:13:25.720+0000

• Duration: 88 min 27 sec

• Commit: e350f16

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Test	Results
Failed	0
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Skipped	1641
Total	25808

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