Replace golang.org/x/exp with stdlib

[Juneezee]

These experimental packages are now available in the Go standard library since Go 1.21.

1. golang.org/x/exp/slices -> slices (https://go.dev/doc/go1.21#slices)
2. golang.org/x/exp/maps -> maps (https://go.dev/doc/go1.21#maps)

[idanovinda]

👍

[FxKu]

👍

[zalando-robot]

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[Juneezee]

Hi @FxKu, the previous E2E job run failed, but it looks like a flaky error. Could you please trigger the CI job again? Thanks

https://github.com/zalando/postgres-operator/actions/runs/17800579342/job/50610445500#step:7:1076

======================================================================
FAIL: test_enable_disable_connection_pooler (test_e2e.EndToEndTestCase)
For a database without connection pooler, then turns it on, scale up,
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/timeout_decorator/timeout_decorator.py", line 82, in new_function
    return function(*args, **kwargs)
  File "/tests/test_e2e.py", line 776, in test_enable_disable_connection_pooler
    self.eventuallyEqual(lambda: k8s.get_deployment_replica_count(), 2,
  File "/tests/test_e2e.py", line 43, in eventuallyEqual
    self.assertEqual(y, x, m.format(y))
AssertionError: None != 2 : Deployment replicas is 2 default

[Juneezee]

@FxKu Another flaky error. Please re-run the job again. Thanks 🙏

https://github.com/zalando/postgres-operator/actions/runs/17815054746/job/50690620177?pr=2857#step:7:1724

======================================================================
FAIL: test_multi_namespace_support (test_e2e.EndToEndTestCase)
Create a customized Postgres cluster in a non-default namespace.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/timeout_decorator/timeout_decorator.py", line 82, in new_function
    return function(*args, **kwargs)
  File "/tests/test_e2e.py", line 1467, in test_multi_namespace_support
    k8s.wait_for_pod_start("spilo-role=master", self.test_namespace)
  File "/tests/k8s_api.py", line 107, in wait_for_pod_start
    time.sleep(self.RETRY_TIMEOUT_SEC)
  File "/usr/local/lib/python3.8/dist-packages/timeout_decorator/timeout_decorator.py", line 69, in handler
    _raise_exception(timeout_exception, exception_message)
  File "/usr/local/lib/python3.8/dist-packages/timeout_decorator/timeout_decorator.py", line 45, in _raise_exception
    raise exception()
timeout_decorator.timeout_decorator.TimeoutError: 'Timed Out'

[Juneezee]

@FxKu @idanovinda The tests finally passed! This PR requires two more approvals again. Would you mind re-approving it when you have some time? 🙏