Upgrade headers to versioned API 202208

[alexander-zierhut]

The Linkedin API now uses versioning and will deprecate the legacy version on Feb, 28.

I have migrated using setApiHeaders, but so not everyone has to read up on this themselves, I am creating this PR.

What I have tested already:

• Create a posts
• Upload assets
• Login with Linkedin
• Read profile / group data

Some more testing for deprecations might be needed.

Sources:
https://www.linkedin.com/developers/news/featured-updates/versioning-content-launch
https://learn.microsoft.com/en-us/linkedin/marketing/versioning

For everyone else reading this:

// Change the API endpoint to the new versioned endpoint
$this->client->setApiRoot("https://api.linkedin.com/rest/");

// Set the API headers to use the new versioning
$this->client->setApiHeaders([
    'Content-Type' => 'application/json',
    'x-li-format' => 'json',
    "LinkedIn-Version" => "202208",
]);```

[sammyaxe]

Will this get merged?

[sammyaxe]

@alexander-zierhut feel free to open PR here https://github.com/sammyaxe/linkedin-api-php-client, will make you maintainer aswell

[alexander-zierhut]

@sammyaxe Sure, will do. I guess you used my workaround as well? This repo has become kind of stale. I'd also like a new version which at least still upgrades it's dependencies. If you are serious about this, I'll be switching our projects to this as well.