gh-156289: Update Unicode to 18.0.0 - #157645

Regenerate the Unicode tables with Tools/unicode/makeunicodedata.py and
bump UNIDATA_VERSION to 18.0.0:

  Modules/unicodedata_db.h
  Modules/unicodename_db.h
  Objects/unicodetype_db.h

Also update the UCD version references in the docs (stdtypes.rst,
unicodedata.rst, re.rst, lexical_analysis.rst) and the explanatory
comments in Lib/re/_properties.py, per the checklist at the top of
makeunicodedata.py.

TEMPORARY TODO: the two bidirectional-table asserts in
Tools/unicode/mkstringprep.py are commented:
Lib/stringprep.py must then be regenerated.