Remove deprecated 'version' key from docker-compose.yml

[webchick]

What was changed

One-liner to remove version: '3.8' from the docker-compose.yml file.

Why?

When you run docker-compose up you get this warning in the terminal:

WARN[0000] [PATH_TO_REPO]/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion

It seems harmless, but adds some mild friction to the getting started experience, and can lead people newer to Temporal / Docker to wonder if they're doing something wrong.

As to why this innocuous line is throwing a warning in the first place, according to AI™ it sounds like as of Docker Compose V2+ Docker Compose no longer uses a fixed "spec version" and instead is based on the Compose Specification, a living standard that is continuously updated. In said standard, Version is marked obsolete.

And while we could keep it around for posterity for the sake of people using Docker Compose V1, according to https://docs.docker.com/compose/releases/migrate/ security updates ended for V1 in July 2021, which was quite awhile ago now.

(There are some other things from that migration guide we could incorporate here; for example apparently now it's recommended to use docker compose instead of docker-compose) but I figured I would test the waters with this PR first before diving in further. :)

Checklist

1. How was this tested:

Re-ran docker-compose up as indicated in the project README and confirmed not only that the warning didn't show up anymore, but also that the script worked as intended.

% docker -v
Docker version 28.5.1, build e180ab8

2. Any docs updates needed?

Nope.