[5.x]: App::parseEnv doesn't return booleans

### What happened? ### Description Using `App::parseEnv()` doesn't return a boolean when the value of the env variable is `true` or `false`. Instead, it returns the string `'true'` or `'false'`. ```dotenv FOOBAR=true ``` ```php App::parseEnv('$FOOBAR'); // 'true' ``` The docblock clearly indicates that the function should return boolean: > If the string references an environment variable with a value of `true` or `false`, a boolean value will be returned. This was apparently changed in #18418. However, if `App::env()` returns a `boolean`, it's converted back to a string in `App::parseEnv`: ```php if (is_bool($result)) { return $result ? 'true' : 'false'; } ``` Not sure if that is intended and the docblock is just outdated, or the other way around? I'll just use `App::env` for now which _does_ handle boolean conversion. That said, _this_ method _doesn't_ actually mention that it does that. ### Craft CMS version 5.10.1 ### PHP version _No response_ ### Operating system and version _No response_ ### Database type and version _No response_ ### Image driver and version _No response_ ### Installed plugins and versions -