[SPARK-39861][PYTHON][DOCS] Deprecate Python 3.7 Support

### What changes were proposed in this pull request?

This PR aims to deprecate Python 3.7 support at Apache Spark 3.4.0.

### Why are the changes needed?

Apache Spark 3.4 will be released on February 2023 and will be supported for next 18 months.
- https://spark.apache.org/versioning-policy.html

Before Spark 3.4.1, Python 3.7 is going to reach `End Of Support` on 2023-06-27. Although Apache Spark 3.4 will work with Python 3.7 still for a while, there is no proper and official way to support Python 3.7 from that time.
- https://www.python.org/downloads/

### Does this PR introduce _any_ user-facing change?

Yes, but the users will see only a deprecation warning log and docs.

### How was this patch tested?

Pass the CIs.

Closes #37279 from dongjoon-hyun/SPARK-39861.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

docs/index.md

@@ -40,6 +40,7 @@ source, visit [Building Spark](building-spark.html).
 Spark runs on both Windows and UNIX-like systems (e.g. Linux, Mac OS), and it should run on any platform that runs a supported version of Java. This should include JVMs on x86_64 and ARM64. It's easy to run locally on one machine --- all you need is to have `java` installed on your system `PATH`, or the `JAVA_HOME` environment variable pointing to a Java installation.
 
 Spark runs on Java 8/11/17, Scala 2.12/2.13, Python 3.7+ and R 3.5+.
+Python 3.7 support is deprecated as of Spark 3.4.0.
 Java 8 prior to version 8u201 support is deprecated as of Spark 3.2.0.
 For the Scala API, Spark {{site.SPARK_VERSION}}
 uses Scala {{site.SCALA_BINARY_VERSION}}. You will need to use a compatible Scala version

docs/rdd-programming-guide.md

@@ -106,6 +106,7 @@ so C libraries like NumPy can be used. It also works with PyPy 7.3.6+.
 
 Python 2, 3.4 and 3.5 supports were removed in Spark 3.1.0.
 Python 3.6 support was removed in Spark 3.3.0.
+Python 3.7 support is deprecated as of Spark 3.4.0.
 
 Spark applications in Python can either be run with the `bin/spark-submit` script which includes Spark at runtime, or by including it in your setup.py as:
 

python/pyspark/context.py

@@ -306,6 +306,14 @@ def _do_init(
         self.pythonExec = os.environ.get("PYSPARK_PYTHON", "python3")
         self.pythonVer = "%d.%d" % sys.version_info[:2]
 
+        if sys.version_info[:2] < (3, 8):
+            with warnings.catch_warnings():
+                warnings.simplefilter("once")
+                warnings.warn(
+                    "Python 3.7 support is deprecated in Spark 3.4.",
+                    FutureWarning
+                )
+
         # Broadcast's __reduce__ method stores Broadcast instances here.
         # This allows other code to determine which Broadcast instances have
         # been pickled, so it can determine which Java broadcast objects to