Remove obsolete Docker compose version

[thekaveman]

When running docker compose commands locally, we see the following warning message in the output:

WARN[0000] .../eligibility-server/compose.yml: `version` is obsolete

This is referencing the version: "3.8" line in compose.yml, which apparently can now be removed

Acceptance Criteria

• version is removed from compose.yml
• The appcontainer and devcontainer can still build and start successfully using docker compose commands

Additional context

cal-itp/benefits#2054