[SPARK-31964][PYTHON][FOLLOW-UP] Use is_categorical_dtype instead of deprecated is_categorical

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a small followup of #28793 and proposes to use is_categorical_dtype instead of deprecated is_categorical.

is_categorical_dtype exists from minimum pandas version we support (https://github.com/pandas-dev/pandas/blob/v0.23.2/pandas/core/dtypes/api.py), and is_categorical was deprecated from pandas 1.1.0 (pandas-dev/pandas@87a1cc2).

Why are the changes needed?

To avoid using deprecated APIs, and remove warnings.

Does this PR introduce any user-facing change?

Yes, it will remove warnings that says is_categorical is deprecated.

How was this patch tested?

By running any pandas UDF with pandas 1.1.0+:

import pandas as pd
from pyspark.sql.functions import pandas_udf

def func(x: pd.Series) -> pd.Series:
    return x

spark.range(10).select(pandas_udf(func, "long")("id")).show()

Before:

/.../python/lib/pyspark.zip/pyspark/sql/pandas/serializers.py:151: FutureWarning: is_categorical is deprecated and will be removed in a future version.  Use is_categorical_dtype instead
...

After:

...

[SparkQA]

Test build #130068 has finished for PR 30114 at commit 4b1ef1f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34677/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/34677/

[BryanCutler]

LGTM, thanks for catching this @HyukjinKwon !

[BryanCutler]

merged to master.

[HyukjinKwon]

Thanks, @BryanCutler and @srowen.