Remove obsolete version key from compose files

The top-level version: "2" field is ignored by modern Docker Compose,
which warns about it on every invocation. Drop it from docker-compose.yml
and the compose-dev-backend, compose-dev-frontend and compose-e2e-test
files.

compose-dev-backend.yml

@@ -4,7 +4,6 @@
 #
 # build remark42 docker image - docker compose -f compose-dev-backend.yml build
 # start remark42 service - docker compose -f compose-dev-backend.yml up
-version: "2"
 
 services:
   remark42:

compose-dev-frontend.yml

@@ -4,7 +4,6 @@
 #
 # build remark42 docker image - docker compose -f compose-dev-frontend.yml build
 # start remark42 service - docker compose -f compose-dev-frontend.yml up
-version: "2"
 
 services:
   remark42:

compose-e2e-test.yml

@@ -1,5 +1,4 @@
 # compose for running e2e tests in CI
-version: "2"
 
 services:
   remark42:

docker-compose.yml

@@ -1,5 +1,3 @@
-version: "2"
-
 services:
   remark:
     # remove the next line in case you want to use this Docker Compose file separately