README: Remove mentions about supported Ubuntu and Debian versions

The OBS repository is now available for all supported versions of
Ubuntu and Debian so we don't need to explicitly mention versions
that are supported.

README.md

@@ -27,7 +27,7 @@ Daily builds of _libblockdev_ and _libbytesize_ are also in this repo.
 
 #### OBS repository for Ubuntu and Debian
 
-Packages for Debian (testing and unstable) and Ubuntu (19.04 and newer) are available through the Open Build Service.
+Packages for Debian and Ubuntu are available through the Open Build Service.
 Instructions for adding the repository are available [here](https://software.opensuse.org/download.html?project=home:vtrefny&package=python3-blivet).
 
 #### Copr repository for openSUSE, Mageia and OpenMandriva