chore: add rjain90 to the github mgmt stewards list

[galargh]

Summary

Adds @rjan90 to the github-mgmt stewards team.

Why do you need this?

@rjan90 is often involved in GitHub configuration changes and he would benefit from having merge access in this repository. Example - #102

What else do we need to know?

DRI: myself

Reviewer's Checklist

• It is clear where the request is coming from (if unsure, ask)
• All the automated checks passed
• The YAML changes reflect the summary of the request
• The Terraform plan posted as a comment reflects the summary of the request

[github-actions]

The following access changes will be introduced as a result of applying the plan:

Access Changes

User rjan90:
  - will have the permission to github-mgmt change from triage to push

[github-actions]

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

Terraform plans

filecoin-project

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

Terraform will perform the following actions:

  # github_team_membership.this["github-mgmt stewards:rjan90"] will be created
  + resource "github_team_membership" "this" {
      + etag     = (known after apply)
      + id       = (known after apply)
      + role     = "member"
      + team_id  = "7326115"
      + username = "rjan90"
    }

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