make achingbrain a GitHub Management steward

[galargh]

I'm also removing Gus from Stewards group while I'm at it.

[github-actions]

Before merge, verify that all the following plans are correct. They will be applied as-is after the merge.

Terraform plans

ipfs

Terraform used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
  + create
  - destroy

Terraform will perform the following actions:

  # github_team_membership.this["github-mgmt stewards:achingbrain"] will be created
  + resource "github_team_membership" "this" {
      + etag     = (known after apply)
      + id       = (known after apply)
      + role     = "member"
      + team_id  = "6421993"
      + username = "achingbrain"
    }

  # github_team_membership.this["github-mgmt stewards:guseggert"] will be destroyed
  # (because key ["github-mgmt stewards:guseggert"] is not in for_each map)
  - resource "github_team_membership" "this" {
      - etag     = "W/\"774fa934b16f84c657aabfc37fe75705f0364587f8db40c3dd244801080e06b1\"" -> null
      - id       = "6421993:guseggert" -> null
      - role     = "member" -> null
      - team_id  = "6421993" -> null
      - username = "guseggert" -> null
    }

Plan: 1 to add, 0 to change, 1 to destroy.