tcp_proxy: remove tcp_proxy_set_idle_timer_immediately_on_new_connection runtime guard

[wdauchy]

Commit Message:
This flag was added in 1.36.0 and has been defaulting to true for multiple releases. The idle timer is now always started immediately on new connections.
Additional Description:
Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #44076 was opened by wdauchy.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #44076 was ready_for_review by wdauchy.

see: more, trace.

[wdauchy]

Removing tcp_proxy_set_idle_timer_immediately_on_new_connection folds everyone onto the immediate idle-timer path; changelog entry documents the dropped runtime knob. nit: anyone who forced the guard off loses that escape hatch—release notes are the only migration signal.

Thanks for the review! The previous behavior (not starting the idle timer immediately) was an actual bug that caused connection leaks when the upstream connection wasn't fully established. In my own opinion, there's no legitimate reason to keep the old code path as disabling the guard would mean opting back into a known resource leak(?).

This follows the standard Envoy process for runtime guard removal: the fix shipped in 1.36.0, has been defaulting to true for 2 releases, and the changelog entry documents the removal for anyone tracking runtime flag changes.

do you have other concerns?