test: disable test_readonly_node_gc until proper fix (#9755)

## Problem

After investigation, we think to make `test_readonly_node_gc` less
flaky, we need to make a proper fix (likely involving persisting part of
the lease state). See https://github.com/neondatabase/neon/issues/9754
for details.

## Summary of changes

- skip the test until proper fix.

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

test_runner/regress/test_readonly_node.py

@@ -122,6 +122,7 @@ def test_readonly_node(neon_simple_env: NeonEnv):
         )
 
 
+@pytest.mark.skip("See https://github.com/neondatabase/neon/issues/9754")
 def test_readonly_node_gc(neon_env_builder: NeonEnvBuilder):
     """
     Test static endpoint is protected from GC by acquiring and renewing lsn leases.