Debian 11 bullseye DLA-4018-2: ruby2.7 regression fix for XML

A corner case of XML namespace default namespace was not handled
correctly, and thus rexml failed to parse valid XML file.

For Debian 11 bullseye, this problem has been fixed in version
2.7.4-1+deb11u4.

We recommend that you upgrade your ruby2.7 packages.

For the detailed security status of ruby2.7 please refer to
its security tracker page at:
https://security-tracker.debian.org/tracker/source-package/ruby2.7

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at: https://wiki.debian.org/LTS