ElasticSearch 7 is now the major version. Replacing old-style
ES with it by default.
also add basic test for ElasticSearch 7.14, to make sure we will
test ES 7 once we move up to next higher version.
Note: ES 5 is no longer beeing tested as it looks outphased.
see also: https://github.com/rsyslog/rsyslog/pull/4566
