fix(framework): Show update check consistently

[chongshenng]

Description:

The update check message was not reliably shown to users due to two issues:

1. --version flag skipped the update check entirely. For flower-superlink, flower-supernode, and flower-superexec, warn_if_flwr_update_available() was called after parse_args(). Since --version uses argparse's action="version", it exits during parse_args() before the update check runs — meaning the cache file is never even created.
2. The 12-hour display throttle suppressed the message on re-runs. After showing the message once, last_shown_at was recorded and subsequent invocations within 12 hours were silenced. This meant flower-superlink --insecure would generate the cache on the first run but not show the message on re-run.

Changes:

1. Move warn_if_flwr_update_available() before parse_args() in run_superlink(), flower_supernode(), and flower_superexec() so the update check runs even for --version.
2. Remove the 12-hour display throttle — the update message now prints on every invocation when a cached update is available.
3. Remove _mark_cached_flwr_update_message_shown() and all last_shown_at bookkeeping (no longer needed).
4. Update tests to reflect the new always-show

[chongshenng]

Closed in favor of #6939 6938