Use non-deprecated license identifier

[adamruzicka]

CI on ruby 3.3 started failing with

WARNING:  License identifier 'GPL-3.0' is deprecated. Use an identifier from
https://spdx.org/licenses or 'Nonstandard' for a nonstandard license,
or set it to nil if you don't want to specify a license.
Did you mean 'AFL-3.0', 'APL-1.0', 'CPL-1.0', 'EPL-1.0', 'EPL-2.0', 'IPL-1.0', 'LPL-1.0', 'MPL-1.0', 'MPL-2.0', 'NPL-1.0', 'OPL-1.0', 'OSL-3.0', 'QPL-1.0', 'SPL-1.0', 'TPL-1.0', 'UPL-1.0', 'YPL-1.0', 'ZPL-2.0'?
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)