CI: Update Debian releases

• Disable MSVC CI by default for now

  The necessary CI runner doesn't seem to be available, and we shouldn't let that prevent us from merging dbus changes. This can be reverted when the runner comes back.

• CI: Remove EOL versions of Debian and Ubuntu

  Debian 9 'stretch' is EOL, and Ubuntu 16.04 is EOL unless you enter into a special subscription with Canonical. This puts them outside our informal security-support policy, and realistically, anyone sufficiently change-averse to be following these distributions is not going to be backporting a current version of dbus.

• CI: Build on Debian 11 'bullseye' instead of Debian 10 'buster'

  This is the current stable release for Debian, making Debian 10 a much less interesting target for backports. Add a manually-triggered job so we can still try buster occasionally.

  Continue to use buster for mingw-w64 builds until format string issues with bullseye toolchains can be sorted out.

• CI: Install ci-certificates

  We need this to be able to download MSYS packages now.

• CI: Use https to download MSYS packages