Flip --incompatible_allow_python_version_transitions

With this flag enabled by default, the Python version is no longer "sticky" and forced to a fixed value by a command line flag. Instead it adjusts to whatever the `py_binary` or `py_test` declares its version to be.

For more information see feature tracking issue #6583 and flag migration tracking issue #7307.

Closes #7307.

RELNOTES[INC]: (Python rules) The python version now changes to whatever version is specified in a `py_binary` or `py_test`'s `python_version` attribute, instead of being forced to the value set by a command line flag. You can temporarily revert this change with `--incompatible_allow_python_version_transitions=false`. See [#7307](#7307) for more information.

PiperOrigin-RevId: 240227319

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

@@ -87,7 +87,7 @@ public String getTypeDescription() {
 
   @Option(
       name = "incompatible_allow_python_version_transitions",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {