Add open-ended bc-version range shorthand [N..]

A closed range like [23..28] wrongly implies guidance stops applying after
BC28, so a reviewer targeting BC29+ would not match the file. Introduce an
open-ended shorthand [N..] meaning ''version N and every later version''.

- validate_frontmatter.py: RANGE_SHORTHAND allows an optional upper bound;
  expand_bc_version returns the normalized string ''N..'' for open-ended.
- read.md: document the fourth bc-version form and its matching rule
  (matches target >= N; not enumerable).
- write.md: prefer [N..] over a closed range for a feature introduced in N
  and not expected to be removed.
- Apply [23..] to the actionable-errors article (actionable errors shipped
  in BC23 and are not version-bounded above).
- README: mention [N..] in the frontmatter example.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/error-handling/prefer-errorinfo-for-actionable-errors.md

@@ -1,5 +1,5 @@
 ---
-bc-version: [23..28]
+bc-version: [23..]
 domain: error-handling
 keywords: [errorinfo, actionable-errors, fix-it, show-it, addaction, addnavigationaction, error-dialog]
 technologies: [al]