chore(deps): bump typescript to 6.0.3

typescript-eslint (pulled in transitively via @wordpress/eslint-plugin)
only supports typescript >=4.8.4 <6.1.0, even on its latest release, so
6.0.3 is the highest jointly-supported version - not the nominal
latest (7.0.2), which breaks lint:js outright:

  typescript-eslint does not support TS 7.0.

Verified compile:js and test:unit pass; lint:js pre-existing errors
(unrelated to this bump) are unchanged.

Author: holyhope