Revert "install_pmbootstrap.sh: use 2.3.x branch (MR 6)"

pmbootstrap master is now good enough to use as default in CI.

See https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2396

This reverts commit 755b5064.

install_pmbootstrap.sh

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 # usage: install_pmbootstrap.sh [ADDITIONAL_PACKAGE, ...]
 
-: "${PMBOOTSTRAP_TAG:="2.3.x"}"
+: "${PMBOOTSTRAP_TAG:="master"}"
 : "${PMBOOTSTRAP_URL:="https://gitlab.com/postmarketOS/pmbootstrap.git"}"
 
 # Add the "origin-original" remote to an existing pmaports.git clone, so