Updating the OS matrix for CI

Updating the OS matrix for CI

PRs:
* Remove Debian 8 (not supported in 3.0)
* Add Debian 9 (minimum supported Debian in 3.0)
* Remove openSUSE 42 (end of support this month)
* Add SLES 15 (best upgrade for openSUSE 42)
* Remove Fedora 28 (end of support)
* Add Fedora 29 (best supported version)
* Remove macOS 10.12 (not supported in 3.0, effective end of support)

Full Runs:
* Remove Debian 8 (not supported in 3.0)
* Remove Ubuntu 18.10 (end of support next month)
* Remove openSUSE 42 (end of support this month)
* Remove Fedora 28 (end of support)
* Remove macOS 10.12 (not supported in 3.0, effective end of support)
* Add Windows 10 1903 ("19H1") (current release)

Commit migrated from https://github.com/dotnet/corefx/commit/dda4588873cde2740d15e32749ccfd13eb33db07