fix: make executable implicit defaults public so other modules can use rule builders (#3919) (#3970)

Backports #3919 to `release/1.9` (requested by @aignas / @rickeylev).

`create_executable_rule_builder()` (used by `py_binary_rule_builder()` /
`py_test_rule_builder()`, the public `python/api/executables.bzl` API)
bundles three implicit attrs whose defaults point at private targets
under
`//python/private`. These had no explicit visibility, so any external
module constructing a rule via the builder API failed at analysis time
with a visibility error.

Commits:
- `fix: make executable implicit defaults public ... (#3919)` —
cherry-pick
  of `19ffd215`, unchanged from main.
- `Release 1.9.2: Update changelog` — finalizes the changelog entry
(`release/1.9` doesn't carry the `news/` fragment infra main uses, so
the
fragment is folded into `CHANGELOG.md` directly, matching
#3775/#3776/#3819).

Fixes #3919 on `1.9.x`.

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>