[JENKINS-75174] Move existing web.xml to web-fragment.xml under core

[Vlatombe]

As this impacts only EOM integrators (CloudBees), I deliberately omitted changelog. Please let me know if you think otherwise.

• Depends on [JENKINS-75174] Allow webapp context configuration to be discovered via Servlet 3.0 discovery jenkins-test-harness#899 to keep test module successful.
• Depends on [JENKINS-75174] Ensure PCT uses a version of jth that is compatible with web fragments plugin-compat-tester#743 for BOM

Noting that plugins using a Jenkins version including this change will need to use the jth version mentioned above or later.

See JENKINS-75174.

Testing done

Proposed changelog entries

• N/A

Proposed upgrade guidelines

N/A

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1. The Jira issue, if it exists, is well-described.
2. 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.
3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. 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).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. 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:

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1. There are at least two (2) approvals for the pull request and no outstanding requests for change.
2. Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. 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).
6. 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).

[github-actions]

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

[timja]

Noting that plugins using a Jenkins version including this change will need to use the jth version mentioned above or later.

Does this affect PCT? i.e is a bom build needed?

[Vlatombe]

is a bom build needed ?

Yes, I will need a patch on pct repo as well to pick up jenkinsci/jenkins-test-harness#899

[jglick]

Pity that Git rename detection fails when you keep a relatively unrelated file under the original name.

[github-actions]

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

[Vlatombe]

This change would need a BOM build with jenkinsci/plugin-compat-tester#746