Disable SSL on testing old BWC nodes (#30337)

Old versions of Elasticsearch with SSL have a problem due to a bug in
Netty where an illegal reference count exception can cause the cluster
to fall apart. This issue impacted Netty 4.1.5.Final and
4.1.6.Final. With:

$ for tag in $(git tag | grep -E 'v5\.[0-9]\.[0-9]$')
> do
>   echo -n "$tag " \
>     && git show $tag:modules/transport-netty4/build.gradle \
>     | grep netty-buffer \
>     | cut -d: -f3 \
>     | perl -p -e 's/"$//g'
> done
v5.0.0 4.1.5.Final
v5.0.1 4.1.5.Final
v5.0.2 4.1.5.Final
v5.1.1 4.1.6.Final
v5.1.2 4.1.6.Final
v5.2.0 4.1.7.Final
v5.2.1 4.1.7.Final
v5.2.2 4.1.7.Final
v5.3.0 4.1.7.Final
v5.3.1 4.1.7.Final
v5.3.2 4.1.7.Final
v5.3.3 4.1.7.Final
v5.4.0 4.1.9.Final
v5.4.1 4.1.11.Final
v5.4.2 4.1.11.Final
v5.4.3 4.1.11.Final
v5.5.0 4.1.11.Final
v5.5.1 4.1.11.Final
v5.5.2 4.1.11.Final
v5.5.3 4.1.11.Final
v5.6.0 4.1.13.Final
v5.6.1 4.1.13.Final
v5.6.2 4.1.13.Final
v5.6.3 4.1.13.Final
v5.6.4 4.1.13.Final
v5.6.5 4.1.13.Final
v5.6.6 4.1.13.Final
v5.6.7 4.1.13.Final
v5.6.8 4.1.13.Final
v5.6.9 4.1.13.Final

We see that this means we need to not use SSL when testing nodes before
5.2.0. That is what this commit does.

x-pack/qa/full-cluster-restart/build.gradle

@@ -163,7 +163,7 @@ subprojects {
       setting 'logger.level', 'DEBUG'
 
       setting 'xpack.security.enabled', 'true'
-      setting 'xpack.security.transport.ssl.enabled', 'true'
+      setting 'xpack.security.transport.ssl.enabled', version.onOrAfter("5.2.0").toString()
       setting 'xpack.ssl.keystore.path', 'testnode.jks'
       setting 'xpack.ssl.keystore.password', 'testnode'
       if (version.onOrAfter('6.3.0')) {