chore: mark old updater test script as deprecated

[ev0r0x]

What changed

The test script tests/test_updater.sh relied on an old auto-update mechanism that no longer exists.
This includes the --updater-mode test flag and files like .prover.pid and .current_version.

To prevent confusion, the script is now explicitly marked as DEPRECATED at the top:

# DEPRECATED: This test relies on the old updater logic and does not run in CI.
# It is kept for historical reference only.

Why

Leaving the script unmarked could mislead developers into thinking the auto-update is still tested by this script.
This change keeps the history but clearly signals that it shouldn’t be run.

Notes

• The functional code has not changed.
• Initially, the possibility of completely removing the script was considered, since it no longer performs its function,
  but it was decided to simply add a description.