fix: can't change and override valid locales

[kenjis]

Description
Fixes #4297

• add IncomingRequest::setValidLocales()

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

An elegant solution.

[kenjis]

Added docs.

[iRedds]

Of course this is a bit late, but what does request state have to do with locales?
The incoming request either contains the locale or does not.
I think the request class should not be concerned with locale validation.

[kenjis]

Now that we have added setValidLocales(), we can change the locale to any value later.
However, there is still room for improvement in where the locale of the request is determined.

I found that we still cannot change valid locales for content negotiation:

CodeIgniter4/system/HTTP/IncomingRequest.php

Line 220 in e1c13f6

$this->setLocale($this->negotiate('language', $config->supportedLocales));

in the constructor:

CodeIgniter4/system/HTTP/IncomingRequest.php

Line 179 in e1c13f6

$this->detectLocale($config);