Drop symfony4 support

[sral97]

• Use new release of php-rest-api
• Remove dev-dependency to sensio/framework-extra-bundle
• Upgrade other dependencies as well

[Edwin-Luijten]

Thanks a bunch @sral97 , I tested your pull request and saw that the required monolog version requires 8.1+.

IMO It's safe to bump the version of php to 8.1 (8.0 is EO Security fixes only and EOL in 10 month's anyway).
It's also possible to support both 8.0 and 8.1, only need to apply a second version constraint on monolog, that supports 8.0.

Also, just like with the php-rest-api package it's best to also support symfony ^6.

I'm also thinking of dropping the suggestion of using the jms/serializer-bundle.
And I will change from travis to github workflow, updating the php version matrix to use supported versions after your contributions are merged (Thanks again).

[sral97]

I didn't add the symfony 6 support in this PR because that would require to update the functional tests to work with symfony 6. I would treat that as a seperate issue/PR, as I was not sure if the kernel can be written in a way to support both symfony 5 and 6. I should be able to look into this sometime this week.

[sral97]

Good news: it looks like all that was needed to get it working with both symfony versions was to drop the explicit route loading in the kernel and just rely on the one provided by each versions' MicroKernelTrait.