CI: Remove testing pyarrow 6 (#46183)

Author: mroeschke

.github/workflows/posix.yml

@@ -28,7 +28,7 @@ jobs:
         pattern: ["not single_cpu", "single_cpu"]
         # Don't test pyarrow v2/3: Causes timeouts in read_csv engine
         # even if tests are skipped/xfailed
-        pyarrow_version: ["5", "6", "7"]
+        pyarrow_version: ["5", "7"]
         include:
           - env_file: actions-38-downstream_compat.yaml
             pattern: "not slow and not network and not single_cpu"