Flip --incompatible_remove_old_python_version_api

With this flag enabled by default, it is no longer allowed to set the `default_python_version` attribute or `--force_python` flag; use the `python_version` attribute and `--python_version` flag respectively instead.

For more information see feature tracking issue bazelbuild#6583 and flag migration tracking issue bazelbuild#7308.

Closes bazelbuild#7308.

TESTED=https://buildkite.com/bazel/bazelisk-plus-incompatible-flags/builds/51

RELNOTES[INC]: (Python rules) The `default_python_version` attribute of the `py_binary` and `py_test` rules has been renamed to `python_version`. Also, the `--force_python` flag has been renamed to `--python_version`. See [bazelbuild#7308](bazelbuild#7308) for more information.

PiperOrigin-RevId: 239600472

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

@@ -71,7 +71,7 @@ public String getTypeDescription() {
 
   @Option(
       name = "incompatible_remove_old_python_version_api",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {