fix: Give Atlantis write permission to EFS

[showwin]

Description

Add elasticfilesystem:ClientWrite permission to the EFS file system policy.

Motivation and Context

Currently, we give only the mounting permission, so Atlantis ECS container can mount/read EFS, but cannot write anything under /home/atlantis/* directory. Due to this problem, Atlantis container cannot start with enable_efs = true option.
This is the error message I saw in the Atlantis log.

Error: initializing server: unable to create dir "/home/atlantis/.atlantis/bin": mkdir /home/atlantis/.atlantis: read-only file system

The error was solved when I added the elasticfilesystem:ClientWrite permission.

The current workaround is to configure efs parameter as follows:

enable_efs = true
efs = {
  mount_targets = {
    "eu-west-1a" = {
      subnet_id = module.vpc.private_subnets[0]
    }
    ...
  }
  attach_policy = false # <= Here
}

Breaking Changes

N/A

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[bryantbiggs]

I'm surprised this wasn't caught before, thank you!

[antonbabenko]

This PR is included in version 4.0.4 🎉

[github-actions]

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