Add feature check for UTF-8 Stardoc input.

bazelbuild/bazel#24935 changes the observable behavior
of starlark_doc_extract, and consumers need to adapt.

Work towards bazelbuild/bazel#374
Work towards phst/rules_elisp#818

features.bzl

@@ -40,6 +40,10 @@ _docs = struct(
     # The stardoc output changed in https://github.com/bazelbuild/bazel/commit/bd1c3af2ea14e81268e940d2b8ba5ad00c3f08d7
     # This may be required for "diff tests" that assert on the generated API docs.
     kwargs_name_with_double_star = ge("8.0.0-pre.20240603.2"),
+    # Starting with Bazel 8.1.0, all input to Stardoc is interpreted as UTF-8.
+    # Previously, it was interpreted as Latin-1.  See
+    # https://github.com/bazelbuild/bazel/pull/24935.
+    utf8 = ge("8.1.0"),
 )
 
 _external_deps = struct(