fix: unify casing in BaseService::injectMock

[ThomasMeschke]

Description
The BaseService::injectMock method only applied the strtolower function to the name in regard to the mock-list, but not to the instance-list:

public static function injectMock(string $name, $mock)
{
    static::$instances[$name]         = $mock;
    static::$mocks[strtolower($name)] = $mock;
}

Every other method in the BaseService applies strtolower to the name argument before doing anything else, e.g. resetSingle:

public static function resetSingle(string $name)
{
    $name = strtolower($name);
    unset(static::$mocks[$name], static::$instances[$name]);
}

... or getSharedInstance:

protected static function getSharedInstance(string $key, ...$params)
{
    $key = strtolower($key);

    // Returns mock if exists
    if (isset(static::$mocks[$key])) {
        return static::$mocks[$key];
    }

    if (! isset(static::$instances[$key])) {
        // Make sure $getShared is false
        $params[] = false;

        static::$instances[$key] = AppServices::$key(...$params);
    }

    return static::$instances[$key];
}

This results in two different instance entries when injectMock gets called with "fooBar" and with "foobar" respectively, but only one of them (namely "foobar") is ever to be resolved or reset again.
When not using all-lowercase while calling injectMock, any existing instance that is meant to be replaced is therefore not being replaced.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Thank you. Could you add a changelog entry here: https://github.com/codeigniter4/CodeIgniter4/blob/develop/user_guide_src/source/changelogs/v4.7.4.rst