Propose to add the new security section to the Beam releases

[XQ Hu via dev]

Hi All,

We are proposing to explicitly add the security fixes to the Beam release
notes. https://github.com/apache/beam/pull/27976 modified the template
in CHANGES.md
by adding this new section.

Please let us know if you have any questions or feel free to comment PR:
https://github.com/apache/beam/pull/27976.

Thanks.

Best,
XQ