Fix license identifier

According to <https://spdx.org/licenses/>. `gem build` requires this
now.

schleuder-cli.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |s|
   # TODO: extend/replace expired cert
   #s.signing_key = "#{ENV['HOME']}/.gem/schleuder-gem-private_key.pem"
   #s.cert_chain  = ['gem-public_cert.pem']
-  s.license = 'GPL-3.0'
+  s.license = 'GPL-3.0-only'
   s.add_runtime_dependency 'thor', '~> 1'
   if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.3.0")
     s.add_runtime_dependency 'base64', '~> 0.2.0'