[JENKINS-36718] Specify charset in DM_DEFAULT_ENCODING paths

[Flamki]

Fixes #17339

Testing done

• Replaced default-encoding usage with explicit charset handling in these core paths:
  • TextFile.head(int) now reads with UTF-8.
  • WindowsServiceLifecycle.restart() now decodes launcher output with task.getCharset().
  • ConsoleNote.encodeTo(Writer) and ConsoleNote.encode() now decode bytes as UTF-8.
• Removed now-unneeded @SuppressFBWarnings("DM_DEFAULT_ENCODING") suppressions in the touched methods.
• Local Maven execution is not available in this environment (mvn and mvnw are unavailable), so local test execution was not possible here. CI will provide full validation.

Screenshots (UI changes only)

Before

N/A (no UI change)

After

N/A (no UI change)

Proposed changelog entries

• Specify explicit character sets in core text encoding paths to avoid default-encoding behavior.

Proposed changelog category

/label internal

Proposed upgrade guidelines

N/A

Submitter checklist

• The 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.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate. (No new public API in this PR.)
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable. (No new deprecations in this PR.)
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, 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). (No UI/JavaScript changes in this PR.)
• For dependency updates, there are links to external changelogs and, if possible, full differentials. (No dependency updates in this PR.)
• For new APIs and extension points, there is a link to at least one consumer. (No new APIs or extension points in this PR.)

Desired reviewers

@jenkinsci/core-pr-reviewers

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).
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[MarkEWaite]

Closing in preference to the earlier pull request:

• Issue Jenkins-36718 - Fix DM_DEFAULT_ENCODING scanned by spotbugs #9784