Drop support for remoting releases before 2024-08

[daniel-beck]

The fix for SECURITY-3430 retains compatibility with older remoting releases to have a smoother upgrade experience. This PR drops that support. It's probably going to be a while before we want to merge this, but I thought it would be useful to have this prepared.

Testing done

Autotests

Proposed changelog entries

• Drop support for Remoting (agent.jar) releases from before August 2024.

Proposed upgrade guidelines

(Copied from #9440 for consistency)

Increase the minimum required Remoting version to 3256.3258.v858f3c9a_f69d (released on August 7, 2024). When an agent with a Remoting version older than 3256.3258.v858f3c9a_f69d connects to the Jenkins controller, the agent connection is rejected. Ensure that all agents are running a recent version of Remoting prior to upgrading. Agents with unsupported Remoting versions can be allowed to connect to the controller by setting the hudson.slaves.SlaveComputer.allowUnsupportedRemotingVersions system property to true.

### 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](https://github.com/jenkins-infra/jenkins.io/blob/master/content/_data/changelogs/weekly.yml)). 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.
- [ ] New public classes, fields, and methods are annotated with `@Restricted` or have `@since TODO` Javadocs, as appropriate.
- [ ] 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](https://www.jenkins.io/doc/developer/security/csp/)).
- [ ] For dependency updates, there are links to external changelogs and, if possible, full differentials.
- [ ] For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

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

### Maintainer checklist
- [ ] 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.
- [ ] Changelog entries in the pull request title and/or **Proposed changelog entries** are accurate, human-readable, and in the imperative mood.
- [ ] Proper changelog labels are set so that the changelog can be generated automatically.
- [ ] 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](https://github.com/jenkinsci/jenkins/pull/4387)).
- [ ] 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](https://issues.jenkins.io/issues/?filter=12146)).

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!