Fix to update object_url_slug to correct character set and collate

What was the issue? 

Problem was that the object URL slug was using old utf8 encoding and because of that, some slugs available in utf8mb4 weren’t supported. 

How did you solve it? 

Encoding was changed from utf8 to utf8mb4 to match the rest of the database.

Why is this fix important?

Because now user can set URL slug to anything he wants that is within length limitation of MySQL.

In your opinion, what's the biggest advantage that Pimcore offers and other platforms don't? 

I don’t have much experience with other platforms so this is kinda hard to answer, but the biggest advantage I see Pimcore provides is the versatility it has. 

And if you want to check out this pull request, just follow the link!