Use SPDX AGPL license identifier

[bkmgit]

Describe the bug

When building the Gem from source, one gets a warning:

WARNING:  License identifier 'AGPL-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'?

The standard identifiers are:

• AGPL-3.0-only
• AGPL-3.0-or-later
  see:
• https://spdx.org/licenses/AGPL-3.0-only.html
• https://spdx.org/licenses/AGPL-3.0-or-later.html

To Reproduce

git clone https://github.com/decidim/decidim
cd decidim
bundle install
gem build decidim.gemspec

Expected behavior

No warning should be given.

Screenshots

No response

Stacktrace

No response

Extra data

• Device: Server
• Device OS: FreeBSD
• Browser: N/A
• Decidim Version: commit 96c1db0
• Decidim installation: N/A

Additional context

Happy to make a pull request to update the relevant files:
./decidim-accountability/decidim-accountability.gemspec: s.license = "AGPL-3.0"
./decidim-admin/decidim-admin.gemspec: s.license = "AGPL-3.0"
./decidim-api/decidim-api.gemspec: s.license = "AGPL-3.0"
./decidim-assemblies/decidim-assemblies.gemspec: s.license = "AGPL-3.0"
./decidim-blogs/decidim-blogs.gemspec: s.license = "AGPL-3.0"
./decidim-budgets/decidim-budgets.gemspec: s.license = "AGPL-3.0"
./decidim-comments/decidim-comments.gemspec: s.license = "AGPL-3.0"
./decidim-conferences/decidim-conferences.gemspec: s.license = "AGPL-3.0"
./decidim-core/decidim-core.gemspec: s.license = "AGPL-3.0"
./decidim-debates/decidim-debates.gemspec: s.license = "AGPL-3.0"
./decidim-design/decidim-design.gemspec: s.license = "AGPL-3.0"
./decidim-dev/decidim-dev.gemspec: s.license = "AGPL-3.0"
./decidim-forms/decidim-forms.gemspec: s.license = "AGPL-3.0"
./decidim-generators/decidim-generators.gemspec: s.license = "AGPL-3.0"
./decidim-generators/lib/decidim/generators/component_templates/decidim-component.gemspec.erb: s.license = "AGPL-3.0"
./decidim-generators/lib/decidim/generators/component_templates/decidim-component.gemspec.erb: f.start_with?(*%w(app/ config/ db/ lib/ LICENSE-AGPLv3.txt Rakefile README.md))
./decidim-generators/lib/decidim/generators/component_templates/package.json.erb: "license": "AGPL-3.0",
./decidim-generators/spec/runtime/component_generator_spec.rb: "license" => "AGPL-3.0",
./decidim-generators/spec/runtime/path_flag_spec.rb: "license" => "AGPL-3.0",
./decidim-initiatives/decidim-initiatives.gemspec: s.license = "AGPL-3.0"
./decidim-meetings/decidim-meetings.gemspec: s.license = "AGPL-3.0"
./decidim-pages/decidim-pages.gemspec: s.license = "AGPL-3.0"
./decidim-participatory_processes/decidim-participatory_processes.gemspec: s.license = "AGPL-3.0"
./decidim-proposals/decidim-proposals.gemspec: s.license = "AGPL-3.0"
./decidim-sortitions/decidim-sortitions.gemspec: s.license = "AGPL-3.0"
./decidim-surveys/decidim-surveys.gemspec: s.license = "AGPL-3.0"
./decidim-system/decidim-system.gemspec: s.license = "AGPL-3.0"
./decidim-templates/decidim-templates.gemspec: s.license = "AGPL-3.0"
./decidim-verifications/decidim-verifications.gemspec: s.license = "AGPL-3.0"
./decidim.gemspec: s.license = "AGPL-3.0"
./package-lock.json: "license": "AGPL-3.0"
./package-lock.json: "license": "AGPL-3.0",
./package-lock.json: "license": "AGPL-3.0",
./package-lock.json: "license": "AGPL-3.0",
./package-lock.json: "license": "AGPL-3.0",
./package-lock.json: "license": "AGPL-3.0",
./package-lock.json: "license": "AGPL-3.0",
./packages/browserslist-config/package.json: "license": "AGPL-3.0",
./packages/core/package.json: "license": "AGPL-3.0",
./packages/dev/package.json: "license": "AGPL-3.0",
./packages/eslint-config/package.json: "license": "AGPL-3.0",
./packages/prettier-config/package.json: "license": "AGPL-3.0",
./packages/stylelint-config/package.json: "license": "AGPL-3.0",
./packages/webpacker/package.json: "license": "AGPL-3.0",