Backport of Check websocket upgrade headers correctly with multiple values into release/2.0.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #28234 to be assessed for backporting due to the inclusion of the label backport/2.0.x.

The below text is copied from the body of the original PR.

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Description

Some browsers will add additional Connection headers, (keep-alive), and send them as a comma separated list, which won't parse correctly by just using req.Header.Get. Updates the header check to use gorilla/websockets logic to ensure we can support these headers, and add tests to prevent regressions.

Testing & Reproduction steps

Added test to highlight the problem, even though it ends up testing the gorilla/websocket package. It's more to highlight that we should be able to handle these types of headers.

Links

Fixes #28216

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad product documentation, which is stored in the
  web-unified-docs repo. Refer to the web-unified-docs contributor guide for docs guidelines.
  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.
• LLM Usage If an LLM was used to generate any code, please ensure and confirm you have read
  and followed the AI usage guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

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Overview of commits

• f30ea73

• 505c44a