Change platform and Swift version shields.io badge colour

[daveverwer]

Discussed in #3410

Change the colour of the background for the badges to blue to indicate information rather than any other kind of status.

The logo colour should not be changed, only the background.

[daveverwer]

The mid-blue is far too strong 😬

[hassila]

What about the blue at the button you use?

[daveverwer]

That's the same mid-blue, just the dark mode version of it.

Shields.io have a standard "blue". I'm trying that, but it takes a while to iterate as it needs to be deployed to staging every time.