gitpod template comments

Hello!
I'm relatively new to ComsWasm and Gitpod, but I found a number of inconsistencies between the template and gitpod documentation, so I'd like to comment on them and may be I am wrong somewhere, please ignore me then.

### .gitpod.Dockerfile and .gitpod.yml are inconsistent
`.gitpod.yml` says that as a base image `cosmwasm/cw-gitpod-base:v0.16`, so it is not using `.gitpod.Dockerfile`. I think the file should be dropped for consistency.

### Extension `rust-lang.rust` is outdated
VSCode recommends to replace it with rust-lang.rust-analyzer

I'm happy to send a pull request if what I'm saying makes sense

Thank you!

Compare .gitpod.yml with .gitpod.Dockerfile and the Gitpod documentation to confirm which base image is intended. Check the VSCode extension configuration for rust-lang.rust, then update the template so the Dockerfile and extension references are consistent with the documented setup.

Written by the indexing model from the issue text.