add ppa to upgrade gdal (#1892)

* add ppa to upgrade gdal to 3.4 to support obligatory COGs

configure.sh

@@ -58,7 +58,7 @@ ensure_prereqs() {
     if [[ "$UBUNTU_VERSION" == *"20.04"* ]]; then
         echo "Enabling PPA for Ubuntu GIS"
         sudo $APT_GET install -y -qq --no-install-recommends software-properties-common
-        sudo add-apt-repository -y ppa:ubuntugis/ubuntugis-unstable
+        sudo add-apt-repository ppa:ubuntugis/ppa
         sudo $APT_GET update
     fi