python: Flip --incompatible_python_disable_py2 to true

This changes the default to rejecting Python 2 values. For more
information, see #17293

Fixes #17293

RELNOTES[INC]: --incompatible_python_disable_py2 is flipped to true. See #17293 for details.

PiperOrigin-RevId: 507897246
Change-Id: I67cbadbb0f543d8153ad0355af22e48ead9083ef

Author: rickeylev

src/main/java/com/google/devtools/build/lib/rules/python/PythonOptions.java

@@ -121,7 +121,7 @@ public String getTypeDescription() {
 
   @Option(
       name = "incompatible_python_disable_py2",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.INPUT_STRICTNESS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},