doc: update 'asa.rst' for 2.7 release
Update security advisory 2.7 release.
Security Advisory
#################
+Addressed in ACRN v2.7
+************************
+
+We recommend that all developers upgrade to this v2.7 release (or later), which
+addresses the following security issue discovered in previous releases:
+
+-----
+
+- Heap-use-after-free happens in ``MEVENT mevent_handle``
+ The file descriptor of ``mevent`` could be closed in another thread while being
+ monitored by ``epoll_wait``. This causes a heap-use-after-free error in
+ the ``mevent_handle()`` function.
+
+ **Affected Release:** v2.6 and earlier
+
Addressed in ACRN v2.6
************************
