v4.5.8 release

[ChristophWurst]

I'd like to release the two fixes since 4.5.7.

@phil-davis could you tell me how releases are done in this repo? I see that for 4.5.7 changes were forward-ported: 96c3152.

Do the fixes have to be ported to the 4.5 branch?

[phil-davis]

Changes need to be back-ported to the 4.5 branch. I did that just now.

master has other changes related to increasing the phpstan level, which I was doing ages ago but never got finished.

4.5 still supports PHP 7.1 and 7.2! But I might as well keep that support, since it is still easy enough to backport to there and adjust any small bits of code when doing the backports.

I will check now for anything else that look easy to merge to master and then backport. Then I will make a release.

[ChristophWurst]

Alright, thank you!

[phil-davis]

Changelog added in #751

[phil-davis]

https://github.com/sabre-io/vobject/releases/tag/4.5.8

Let me know if there is any problem.