CI: remove macos-13 from CI workflows

macOS 13 has been retired by GitHub Actions. This removes all macos-13
references:

- Delete build-macos-13.yaml workflow
- Delete frontend-macos-13.yaml workflow
- Remove macos-13 from test-docs-scripts.yaml matrix
- Update README.md supported platforms table

Author: dannywillems

.github/workflows/build-macos-13.yaml

@@ -78,7 +78,7 @@ jobs:
     if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch' || contains(github.event.pull_request.labels.*.name, 'test-doc-scripts')
     strategy:
       matrix:
-        os: [macos-13, macos-14, macos-15, macos-latest]
+        os: [macos-14, macos-15, macos-latest]
 
     steps:
       - name: Checkout repository
@@ -203,7 +203,7 @@ jobs:
     if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch' || contains(github.event.pull_request.labels.*.name, 'test-doc-scripts')
     strategy:
       matrix:
-        os: [macos-13, macos-14, macos-15, macos-latest]
+        os: [macos-14, macos-15, macos-latest]
 
     steps:
       - name: Checkout repository
@@ -248,7 +248,7 @@ jobs:
     if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch' || contains(github.event.pull_request.labels.*.name, 'test-doc-scripts')
     strategy:
       matrix:
-        os: [macos-13, macos-14, macos-15, macos-latest, ubuntu-latest]
+        os: [macos-14, macos-15, macos-latest, ubuntu-latest]
 
     steps:
       - name: Checkout repository

.github/workflows/frontend-macos-13.yaml

@@ -141,7 +141,6 @@ Third-party libraries with local modifications. Add vendored dependencies here.
 | ![Ubuntu][ubuntu-icon] **Ubuntu 22.04** | x64           | [![Ubuntu 22.04 x64][ubuntu-22-badge]][ubuntu-22-link]           |
 | ![Ubuntu][ubuntu-icon] **Ubuntu 24.04** | x64           | [![Ubuntu 24.04 x64][ubuntu-24-badge]][ubuntu-24-link]           |
 | ![Ubuntu][ubuntu-icon] **Ubuntu 24.04** | ARM64         | [![Ubuntu 24.04 ARM64][ubuntu-24-arm-badge]][ubuntu-24-arm-link] |
-| ![macOS][macos-icon] **macOS 13**       | Intel         | [![macOS 13 Intel][macos-13-badge]][macos-13-link]               |
 | ![macOS][macos-icon] **macOS 14**       | Apple Silicon | [![macOS 14 M1/M2][macos-14-badge]][macos-14-link]               |
 | ![macOS][macos-icon] **macOS 15**       | Apple Silicon | [![macOS 15 M1/M2/M3][macos-15-badge]][macos-15-link]            |
 | ![macOS][macos-icon] **macOS Latest**   | Apple Silicon | [![macOS Latest][macos-latest-badge]][macos-latest-link]         |
@@ -183,8 +182,6 @@ Third-party libraries with local modifications. Add vendored dependencies here.
   https://github.com/o1-labs/mina-rust/actions/workflows/build-ubuntu-24-04.yaml/badge.svg?branch=develop
 [ubuntu-24-arm-badge]:
   https://github.com/o1-labs/mina-rust/actions/workflows/build-ubuntu-24-04-arm.yaml/badge.svg?branch=develop
-[macos-13-badge]:
-  https://github.com/o1-labs/mina-rust/actions/workflows/build-macos-13.yaml/badge.svg?branch=develop
 [macos-14-badge]:
   https://github.com/o1-labs/mina-rust/actions/workflows/build-macos-14.yaml/badge.svg?branch=develop
 [macos-15-badge]:
@@ -200,8 +197,6 @@ Third-party libraries with local modifications. Add vendored dependencies here.
   https://github.com/o1-labs/mina-rust/actions/workflows/build-ubuntu-24-04.yaml
 [ubuntu-24-arm-link]:
   https://github.com/o1-labs/mina-rust/actions/workflows/build-ubuntu-24-04-arm.yaml
-[macos-13-link]:
-  https://github.com/o1-labs/mina-rust/actions/workflows/build-macos-13.yaml
 [macos-14-link]:
   https://github.com/o1-labs/mina-rust/actions/workflows/build-macos-14.yaml
 [macos-15-link]: