[php-src] Issue #22475: Ability to disable pm.max_children warning

Issue: https://github.com/php/php-src/issues/22475
Author: rmccue

### Description

When fpm reaches the `pm.max_children` value, it issues a warning in the logs of the format:

```
[pool %s] server reached pm.max_children setting (%d), consider raising it
```

([Source 1](https://github.com/php/php-src/blob/ecb2c73cb8632248b6e3948d28f3e4a3e712ddef/sapi/fpm/fpm/fpm_process_ctl.c#L410), [source 2](https://github.com/php/php-src/blob/ecb2c73cb8632248b6e3948d28f3e4a3e712ddef/sapi/fpm/fpm/fpm_process_ctl.c#L429))

This message is clearly designed as a hint to server administrators to review the data to see whether this number needs increasing.

However, in multi-server environments such as ours, we use the fpm queue size as an indicator of backpressure, which we then use to automatically scale our servers horizontally. As such, this warning is unnecessary; in fact, it's basically intentional behaviour in our system design. This is at best _annoying_ in the logs, but it also unnecessarily causes data to be written and pushed throughout error reporting systems.

Worse, this is a `ZLOG_WARNING` rather than `ZLOG_DEBUG`, so it's not suppressed through regular reporting.

Given this is designed as a way to suggest actions to the sysadmin, it would make sense to provide a way to say "please don't warn me" in the configuration.

I'd like to suggest adding a `pm.warn_max_children = no` flag into the php-fpm.conf as a way to intentionally suppress these useless messages.

Alternatively, lowering this to a `ZLOG_NOTICE` (or even `ZLOG_DEBUG`) would solve this issue for us, but I think would have further flow-on effects; in situations where you really *are* resource constrained on a single server, it would be easy to miss the notice.