Remove compatibility with GLib < 2.58

• build: Require GLib 2.58.x

  We now have a backport of GLib 2.58.x in Steam public betas' version of Steam Runtime 1 'scout', and the same version is bundled with relocatable pressure-vessel releases.

  Users of build-aux/many-builds.py will need to run build-aux/many-builds.py deps to download the new scout release before they can continue to compile steam-runtime-tools.

• build: Remove compatibility with GTest not having --tap

  GLib 2.38 supported this, and we now have GLib 2.58 in scout.

• glib-backports: Remove backports from GLib versions before 2.58

  We now require that version.

• glib-backports: Sort by GLib version again

• helpers, launcher: Assume that GDBus provides autocleanup functions

  We now require GLib 2.58, which provides those.

• portal-listener: Always require EXTERNAL authentication

  GLib 2.34 provided the allow-mechanism signal, and we now require 2.58.

• utils: Don't call g_type_init() from the library constructor

  g_type_init() is guaranteed to be called by GLib's own constructor since 2.36, and we now require 2.58.

• tests: Always check for expected warning messages

  GLib 2.32 didn't support this, but 2.34+ does, and we now require 2.58.

/cc @refi64 @denittis