Enable Python toolchains by default

(This is a roll-forward of bf66dc7.)

This flips --incompatible_use_python_toolchains, which deprecates --python_top (and for the most part, --python_path). See #7899 for more on the change and migration procedure.

RELNOTES[INC]: Python rules now determine the Python runtime using toolchains rather than `--python_top` and `--python_path`, which are deprecated. See #7899 for information on declaring Python toolchains and migrating your code. As a side-benefit, this addresses #4815 (incorrect interpreter version used) on non-Windows platforms. You can temporarily opt out of this change with `--incompatible_use_python_toolchains=false`.

Fixes #7899, fixes #7375, significant progress on #4815.

DO NOT SUBMIT: Waiting on fixes for downstream projects, and a bazel breaking change release.

PiperOrigin-RevId: 246409519
Change-Id: I52dd56bac83e74405d713db9f9966daa641c0bae

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

@@ -231,7 +231,7 @@ public String getTypeDescription() {
 
   @Option(
       name = "incompatible_use_python_toolchains",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.GENERIC_INPUTS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {