ci: Revert running CI on pushes to any branch

This cause PRs within the repo to run CI twice.

.github/workflows/docs.yml

@@ -2,6 +2,7 @@ name: Documentation
 
 on:
   push:
+    branches: [main, next]
   pull_request:
     branches: [main, next]
 

.github/workflows/msrv.yml

@@ -2,6 +2,7 @@ name: Rust 1.50
 
 on:
   push:
+    branches: [main, next]
   pull_request:
     branches: [main, next]
 

.github/workflows/nightly.yml

@@ -2,6 +2,7 @@ name: Rust Nightly
 
 on:
   push:
+    branches: [main, next]
   pull_request:
     branches: [main, next]
 

.github/workflows/stable.yml

@@ -2,6 +2,7 @@ name: Rust Stable
 
 on:
   push:
+    branches: [main, next]
   pull_request:
     branches: [main, next]