Fix: egrep is obsolescent, use grep -E

See e.g., egrep(1) [https://linux.die.net/man/1/egrep]:

> two variant programs egrep and fgrep are available. egrep is the same
> as grep -E. fgrep is the same as grep -F. Direct invocation as either
> egrep or fgrep is deprecated

See also the rationale of the POSIX description of grep
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html):

> This grep has been enhanced in an upwards-compatible way to provide
> the exact functionality of the historical egrep and fgrep commands as
> well. It was the clear intention of the standard developers to
> consolidate the three greps into a single command.

Closes: #839

ci-scripts/docker/Dockerfile.cppcheck.xenial

@@ -52,8 +52,8 @@ RUN mkdir -p cmake_targets/log && \
         -I openair3/NAS/COMMON/UTIL \
         -j`nproc` . 2> cmake_targets/log/cppcheck.xml 1> cmake_targets/log/cppcheck_build.txt
 
-RUN egrep -c 'severity="error' cmake_targets/log/cppcheck.xml
+RUN grep -E -c 'severity="error' cmake_targets/log/cppcheck.xml
 
-RUN egrep -c 'severity="warning' cmake_targets/log/cppcheck.xml
+RUN grep -E -c 'severity="warning' cmake_targets/log/cppcheck.xml
 
 RUN cat cmake_targets/log/cppcheck.xml