[rush] Route lockfile-changed warning to stderr (fixes #5406)

[LeSingh1]

Summary

rush list --json corrupts its own JSON output when the lockfile was added or removed in the diff range:

$ rush list --json --impacted-by git:<sha_with_no_lockfile>
Lockfile was created or deleted. Assuming all projects are affected.
{
  "projects": [
     ...
  ]
}

The first line is written to stdout, so the output is no longer parseable by jq or any other tool consuming the JSON.

Fix

Route both branches of the lockfile-status warning in ProjectChangeAnalyzer.ts through terminal.writeWarningLine (which writes to stderr) rather than terminal.writeLine. This is the standard CLI convention for warnings that should not pollute machine-readable stdout, and it matches how the rest of this file already emits non-fatal diagnostics (see existing writeWarningLine calls at lines 418, 446, 458 on main).

iclanton green-lit a PR in the issue thread ("Probably pretty simple to fix. Are you interested in putting together a PR?").

Changeset

Added a patch changeset for @microsoft/rush under common/changes/.

Verification

I do not have a Rush-managed monorepo set up locally to demonstrate the before/after pipe, but the diff is a one-method swap (writeLine → writeWarningLine) and the receiving terminal type already exposes the warning method (used elsewhere in the same file).

Issue

Fixes #5406