GH-136895: Update JIT builds to use LLVM 20 (#140329)

Co-authored-by: Emma Harper Smith <emma@emmatyping.dev>

.github/workflows/jit.yml

@@ -68,7 +68,7 @@ jobs:
           - true
           - false
         llvm:
-          - 19
+          - 20
         include:
           - target: i686-pc-windows-msvc/msvc
             architecture: Win32
@@ -138,7 +138,7 @@ jobs:
       fail-fast: false
       matrix:
         llvm:
-          - 19
+          - 20
     steps:
       - uses: actions/checkout@v4
         with:
@@ -166,7 +166,7 @@ jobs:
       fail-fast: false
       matrix:
         llvm:
-          - 19
+          - 20
     steps:
       - uses: actions/checkout@v4
         with: