Remove 404 Travis image

[petk]

Travis was suspended #15314

[skip ci]

[iluuu1994]

Travis sent me an e-mail a week ago or so that they're aware of the issue and working on it. Not sure what that means in terms of the timeline. The build isn't crucial to us, so I don't object to dropping it. In that case, we should drop all related files though.

[petk]

I've appended two commits here then. First removes the Travis related files and some documentation mentions, and the second removes travis related functionality from tests. Should we go with entire removal here or only files-related removal? Otherwise, Travis can be theoretically re-enabled back if such need comes by looking at this commit(s) or the PHP-8.3 branch.

[iluuu1994]

I don't object to full removal. Travis is one of the more unreliable CI providers. Whether to remove 8.2+ doesn't matter much to me.

[petk]

Merged to master branch then. Thanks everyone.