[8.10](backport #3427) [Bug] Do not fail when remote PGPs are not available

[mergify]

This is an automatic backport of pull request #3427 done by Mergify.
Cherry-pick of b1d2e6b has failed:

On branch mergify/bp/8.10/pr-3427
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit b1d2e6b040.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/fragments/1695035111-Resilient-handling-of-air-gapped-PGP-checks.yaml
	modified:   internal/pkg/agent/application/upgrade/artifact/download/fs/verifier.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/fs/verifier_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/http/verifier.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/verifier.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   testing/integration/upgrade_test.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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[elasticmachine]

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• Start Time: 2023-09-21T14:11:09.453+0000

• Duration: 26 min 39 sec

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Test	Results
Failed	0
Passed	6259
Skipped	57
Total	6316

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.765% (80/81)	👍
Files	67.719% (193/285)	👍
Classes	66.292% (354/534)	👍
Methods	52.978% (1112/2099)	👍
Lines	38.632% (12688/32843)	👍 0.06
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[michalpristas]

Skipping IT as per #3397