[dart2] [client] Bump test package version to >=1.21.6 for dart3 compatability

[0xNF]

fix #17563

The test runner is too outdated to be used for Dart 3 projects. This change updates the test runner to be constrained to >=1.21.6 <1.22.0.

Since this change implies a minimum of Dart 2.18, the header.mustache file is now also bumped from // @dart=2.12 to // @dart=2.18.

I think this is a not inappropriate version update for Dart, since 2.18 has been out for 1.5 years now (August 30, 2022)

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[kuhnroyal]

@wing328