remove master & develop, kept only stable in matrix

Restrict CI workflow to only the main branch for pushes and pull requests.

.github/workflows/ci.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ main, master, develop ]
+    branches: [ main ]
   pull_request:
-    branches: [ main, master, develop ]
+    branches: [ main ]
 
 env: 
   CARGO_TERM_COLOR: always
@@ -17,8 +17,6 @@ jobs:
       matrix:
         toolchain:
           - stable
-          - beta
-          - nightly
     steps:
       - uses: actions/checkout@v4
       - run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}