Provide native Java 11 HTTP client versions of FormValidation#URLCheck methods

[basil]

Building on #7398, provide native Java 11 HTTP client versions of FormValidation#URLCheck methods, deprecating the old HttpURLConnection methods in favor of these and adding test coverage for the new functionality (the old functionality had no test coverage).

Testing done

Ran mvn clean verify -Dtest=hudson.util.FormValidationTest locally. As a bonus, I converted hudson.plugins.git.browser.AssemblaWebDoCheckURLTest#testInitialChecksOnRepoUrl to this new API and verified that doing so chased away the weird Java 19 problem I was seeing in jenkinsci/bom#1637 (comment).

Proposed changelog entries

Provide native Java 11 HTTP client versions of FormValidation#URLCheck methods

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples).
  • Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
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• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
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• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
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• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[basil]

Unrelated change while I was here: while testing with URLs like https://app.assembla.com/login that have a redirect, I noticed that the old HttpURLConnection API automatically followed redirects but the new Java 11 HTTP client API was not. To ease migration I figured it would be best to follow redirects by default. I could not think of a case where you would not want to follow them, and it matches the behavior of the old API. Of course anyone who wants to opt out can always do so by creating a custom builder without this option (something that was not possible with the old API).

[basil]

Minor unrelated change while I was here: append the stack trace for debuggability. We love stack traces.

[basil]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process. Thanks!