Merge #6777: chore: remove GitLab CI configuration file, remove status badge from README

8f86e1b chore: remove GitLab CI configuration file, remove status from README (Kittywhiskers Van Gogh)

Pull request description:

  ## Additional Information

  As we are decommissioning our GitLab CI runners (which made their debut in [dash#3149](#3149) in 2019) in favour of GitHub Actions (which as noted in [dash#6663](#6663) is now at feature-parity with GitLab CI), this pull request removes the `.gitlab-ci.yml` configuration and updates our badges in `README.md` to reflect this change.

  Gone but not forgotten. Sayonara, GitLab CI! 2019-2025.

  ## How Has This Been Tested?

  ![Build Status](https://github.com/user-attachments/assets/8664ca6a-51d4-4138-bc93-b173d461bf63)

  ## Checklist

  - [x] I have performed a self-review of my own code **(note: N/A)**
  - [x] I have commented my code, particularly in hard-to-understand areas **(note: N/A)**
  - [x] I have added or updated relevant unit/integration/functional/e2e tests **(note: N/A)**
  - [x] I have made corresponding changes to the documentation
  - [x] I have assigned this pull request to a milestone _(for repository code-owners and collaborators only)_

ACKs for top commit:
  PastaPastaPasta:
    utACK 8f86e1b
  thephez:
    utACK 8f86e1b
  UdjinM6:
    utACK 8f86e1b

Tree-SHA512: 3dce3de4adfb7b769f8c360699711686c6360e7f48769a0b962d401189ce042f7c398f91d3dcc046d723667ba048732b3d74c5f3ee8fd12e17a19fbfe8124991

Author: PastaPastaPasta

.gitlab-ci.yml

@@ -1,10 +1,9 @@
 Dash Core staging tree
 ===========================
 
-| Source         | `master` | `develop` |
-| -------------- | -------- | --------- |
-| GitHub Actions | [![Build Status](https://github.com/dashpay/dash/actions/workflows/build.yml/badge.svg?branch=master)](https://github.com/dashpay/dash/tree/master) | [![Build Status](https://github.com/dashpay/dash/actions/workflows/build.yml/badge.svg?branch=develop)](https://github.com/dashpay/dash/tree/develop) |
-| GitLab CI      | [![Build Status](https://gitlab.com/dashpay/dash/badges/master/pipeline.svg?key_text=CI&key_width=40)](https://gitlab.com/dashpay/dash/-/tree/master) | [![Build Status](https://gitlab.com/dashpay/dash/badges/develop/pipeline.svg?key_text=CI&key_width=40)](https://gitlab.com/dashpay/dash/-/tree/develop) |
+| `master` | `develop` |
+| -------- | --------- |
+| [![Build Status](https://github.com/dashpay/dash/actions/workflows/build.yml/badge.svg?branch=master)](https://github.com/dashpay/dash/tree/master) | [![Build Status](https://github.com/dashpay/dash/actions/workflows/build.yml/badge.svg?branch=develop)](https://github.com/dashpay/dash/tree/develop) |
 
 https://www.dash.org