Update to ECS 1.4.0 (#14844)

- Update vendored elastic/ecs and fields.ecs.yml from ECS 1.4 branch

Includes elastic/ecs#717 and  elastic/ecs#687 not in released v1.4.0

- Remove field `process.exit_code` from Metricbeat, now in ECS.

CHANGELOG.next.asciidoc

@@ -29,6 +29,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Libbeat: Do not overwrite agent.*, ecs.version, and host.name. {pull}14407[14407]
 - Libbeat: Cleanup the x-pack licenser code to use the new license endpoint and the new format. {pull}15091[15091]
 - Users can now specify `monitoring.cloud.*` to override `monitoring.elasticsearch.*` settings. {issue}14399[14399] {pull}15254[15254]
+- Update to ECS 1.4.0. {pull}14844[14844]
 
 *Auditbeat*
 

NOTICE.txt

@@ -1256,8 +1256,8 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 --------------------------------------------------------------------
 Dependency: github.com/elastic/ecs
-Version: v1.2.0
-Revision: 2eaac192a1ca67edab727d7d9d526c5142ae3eb5
+Version: v1.4.0
+Revision: cc4b36eebec29975f57cd0475c3987c9bde5c15a
 License type (autodetected): Apache-2.0
 ./vendor/github.com/elastic/ecs/LICENSE.txt:
 --------------------------------------------------------------------