Bumps main to version 8.4.0 (#1991)

Bumps to version 8.4.0

Author: picandocodigo

.github/workflows/unified-release.yml

@@ -17,7 +17,7 @@ jobs:
     name: Assemble
     runs-on: ubuntu-latest
     env:
-      STACK_VERSION: "8.3-SNAPSHOT"
+      STACK_VERSION: "8.4-SNAPSHOT"
     steps:
       - name: Checkout
         uses: actions/checkout@v2

elasticsearch/_version.py

@@ -15,4 +15,4 @@
 #  specific language governing permissions and limitations
 #  under the License.
 
-__versionstr__ = "8.3.0"
+__versionstr__ = "8.4.0"