Log Message:
Update to 1.2.21.0. From the changelog:

[Changes contributed by Olly Betts]
- Support passing collapse_max argument to Enquire::set_collapse_key().
  Fixes #669, reported by Felix Ostmann.
- Add missing 'my' to testcases.
- Add 'use strict' to all testcases.
- Add a "LICENSE" section to the main POD.
- META.yml: Specify licence and minimum Perl version.
- Fix t/symboltest.t to work when built against an installed version
  of xapian-core (broken by changes in 1.2.20.0).
- Fix "Filehandle STDOUT reopened as M only for input" warning from
  running "perl Makefile.PL".
- Remove TODO for wrapping MatchAll and MatchNothing - these were added
  in 1.2.13.0.  Reported by Will Crawford.
- Wrap Xapian::BAD_VALUENO.
- Fix "make test" to use the Xapian.so we just built, rather than an
  installed version.