Remove Node.js 6 testing.

- On install tools are run that had deep dependencies that no longer
  work in Node.js 6.
- Due to the age of Node.js 6, the plan to drop support for this old
  version, and the complexities involved in addressing this issue, it
  was easier to just stop the testing.
- If Node.js 6 specific issues are found they will still be addressed as
  needed.

Author: davidlehn

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        node-version: [6.x, 8.x, 10.x, 12.x, 14.x]
+        node-version: [8.x, 10.x, 12.x, 14.x]
     steps:
     - uses: actions/checkout@v2
     - name: Use Node.js ${{ matrix.node-version }}

CHANGELOG.md

@@ -9,6 +9,9 @@
   on some modern browsers. Due to other concerns and the expected minimal
   impact of this change, it is happening in a minor release. Please provide
   feedback if this decision causes problems.
+- Node.js 6 is no longer tested due to development tool dependency updates and
+  to avoid adding additional testing complexity. Node.js 6 will otherwise still
+  be supported until the next major release. Please report any issues found.
 
 ## 3.2.0 - 2020-10-13