Add deprecation notice for the EKS guide

[nandajavarma]

Description

This PR adds a small deprecation notice to the README. We can add a Why section under the deprecation notice if necessary. Some of the tasks left to do still to be done by folks with admin access :

• [ ] Close the issue and pull requests sections in the repo
• [ ] Add a [Deprecated] prefix to the About of the repo
• [x] Add Why to the deprecated notice

Related Issue(s)

Fixes #

How to test

Release Notes

Deprecating the `gitpod-eks-guide` in favor of reference architecture and terraform modules

Documentation