Clarify license to be GPLv3+

Because Aspine has multiple contributors, this change does not go into
effect until it is approved by all copyright holders who have
contributed prior to October 23, 2020, and is merged into the master
branch.

README.md

@@ -176,14 +176,16 @@ Copyright
 [Aspine contributors](https://github.com/Aspine/aspine/graphs/contributors)
 2019–21.
 
-This software is licensed under the GNU General Public License, version 3
-— see the [LICENSE.md](LICENSE.md) file for details.
-
-If you contribute to Aspine, please note that you are consenting to having your
-contributions released under the terms of this license. For any contributions
-dated on or after October 23, 2020, you also consent to having your
-contributions be released under any earlier or later version of the GNU General
-Public License, as published by the Free Software Foundation, if and when the
-licensing terms for Aspine are changed to allow such version(s). (For more
-information about the rationale behind this clause, please read issue
-[#38](https://github.com/Aspine/aspine/issues/38).)
+Aspine is [free/libre](https://www.gnu.org/philosophy/free-sw.html) and
+[open-source](https://opensource.org/osd) software; you can redistribute it
+and/or modify it under the terms of the GNU General Public License as published
+by the Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License (version 3)
+along with this program (see file `LICENSE.md`).  If not, see
+<https://www.gnu.org/licenses/>.

package.json

@@ -6,7 +6,7 @@
   "bugs": {
     "url": "https://github.com/maxtkc/aspine/issues"
   },
-  "license": "GPL-3.0-only",
+  "license": "GPL-3.0-or-later",
   "contributors": [
     "Max Katz-Christy",
     "Cole Killian",