add support back to android 6 marshmallow and up

[fischer-felix]

This adds support for lower android versions, namely everything going back to API 23 (Android 6).

The only thing that prevented this was the use of the BlendModeColorFilter in

• DockerContainerAdapter.kt
• DockerEndpointAdapter.kt
• ManageUsersAdapter.kt

Since the BlendModeColorFilter is the continuation of the since deprecated PorterDuffColorFilter, I just added a check for the API version for every occurrence and use the older filter if it is below 29.

Tested on LG V30, API 28 & Pixel 6 Pro Emulator, API 33

Solves #48

[fischer-felix]

While I was at it, I also modified the store badges in the readme a bit. Now the Google play one is generated with the official badge generator and they are in one line. I had to make the F-Droid badge a bit smaller, since the transparent space around it is slightly larger than with Google play.

Sorry if this is a bit messy now, but I did not want to open a pr for such a small thing.