[7.x] Only prepend scheme to PhpRedis host when necessary

[ryzr]

Description

Since #33871 was released in v7.26.0, there are cases where connections via PhpRedisConnector are broken.

In cases where users may be using a different connector between development/staging/prod, or from blindly copy-pasting config from StackOverflow, the following configuration may be possible:

REDIS_HOST=tls://myproductionredis.cache.amazonaws.com

'default' => [
    'scheme'   => 'tls',
    'host'       => env('REDIS_HOST', '127.0.0.1'),
    'password'   => env('REDIS_PASSWORD', null),
    'port'       => env('REDIS_PORT', 6379),
    'database'   => env('REDIS_DB', 0),
],

The resulting scheme will be tls://tls://myproductionredis.cache.amazonaws.com

Proposed changes

My change uses the Str::start helper to only add the scheme if the host does not start with it already

[GrahamCampbell]

Thanks. Please send this to 6.x.