Update GitPod script or remove it from contributing doc

[yeya24]

Is your proposal related to a problem?

thanos/CONTRIBUTING.md

Line 87 in d3a4684

* Spin up a prebuilt dev environment using Gitpod.io [](https://gitpod.io/#https://github.com/thanos-io/thanos)

We mentioned to spin up a GitPod environment in our contributing doc. However, the gitpod configuration was added 3 years ago and nobody maintains it.

https://cloud-native.slack.com/archives/CL25937SP/p1700335519924849 It caused some confusion for contributors when trying GitPod.

Describe the solution you'd like

There is GH codespace feature now and we have a better maintained devcontainer configuration https://github.com/thanos-io/thanos/tree/main/.devcontainer.

If we still want to maintain GitPod config, we should update GitPod config to be the same as GH codespace. Otherwise, we can remove GitPod config and stop mentioning it in our doc.

Describe alternatives you've considered

(Write your answer here.)

Additional context

(Write your answer here.)

[yeya24]

Resolved by #6907