apacheGH-43933: [CI] Remove docker-compose warnings (apache#43934)

### Rationale for this change

Remove `docker-compose` warnings to avoid confusion when running `docker-compose` and see them appear at the top. The two warnings are about:

- `PYTHON_IMAGE_TAG` not having a default value in `.env`
- `version` in `docker-compose.yml` is obsolete and should be removed.

### What changes are included in this PR?

- Add a `PYTHON_IMAGE_TAG` default value in `.env`
- Remove `version` from `docker-compose.yml`

### Are these changes tested?

Yes, the warnings are removed with these changes.

### Are there any user-facing changes?
 No.

* GitHub Issue: apache#43933

Authored-by: Lysandros Nikolaou <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

.env

@@ -71,6 +71,7 @@ NUMBA=latest
 NUMPY=latest
 PANDAS=latest
 PYTHON=3.8
+PYTHON_IMAGE_TAG=3.8
 R=4.4
 SPARK=master
 TURBODBC=latest

docker-compose.yml

@@ -53,8 +53,6 @@
 #
 # See more in cpp/build-support/run-test.sh::print_coredumps
 
-version: '3.5'
-
 x-common: &common
   GITHUB_ACTIONS: