State the license expression as GPL-3.0-or-later everywhere

[soaressgabriel]

Checklist

• I have read the contributing guidelines
• My code follows the style of this project
• I have added tests that prove my fix is effective (metadata and text only)
• New and existing tests pass locally with my changes
• I have updated the documentation (changelog, README, FAQ)

Description

States the license expression consistently as GPL-3.0-or-later, the SPDX form of the source headers' "version 3 or (at your option) any later version":

• CITATION.cff: license: GPL-3.0-or-later (the deprecated id GPL-3.0 maps to GPL-3.0-only in SPDX); validated with cffconvert --validate.
• README.md: badge label, license paragraph (the link pointed to a nonexistent LICENSE.md), and the license-url reference (master -> main).
• doc/source/faq.rst: the license answer names the "or later" clause and the SPDX identifier.

pyproject.toml and license.rst already carried the expression. The license itself does not change.

Related Issue

Resolve #314

Motivation and Context

Pending decision 5 of the modernization plan, taken by the maintainer on 2026-08-26; noted in #306.

How Has This Been Tested?

cffconvert --validate, pre-commit on the touched files.

Screenshots (if appropriate):

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (709c6d6) to head (fac602d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #316   +/-   ##
=======================================
  Coverage   92.48%   92.48%           
=======================================
  Files          60       60           
  Lines        3911     3911           
  Branches      487      487           
=======================================
  Hits         3617     3617           
  Misses        238      238           
  Partials       56       56           

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