Remove lua51 from CI tests

Lua 5.1 does not support yieldable `pcall` and this limitation prevents running tests

Author: khvzak

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        lua: [lua51, lua52, lua53, lua54, luau]
+        lua: [lua52, lua53, lua54, luau]
     steps:
       - uses: actions/checkout@main
       - name: Cache cargo