chore: update shelljs and drop old node support

No change to logic. This updates the ShellJS peer dependency to the
latest release. This also drops support for all node versions prior to
v18 to match ShellJS's version range, and adds in node v20 and v22.

Test: npm test

Author: nfischer

.github/workflows/main.yml

@@ -10,14 +10,9 @@ jobs:
       fail-fast: false
       matrix:
         node-version:
-          - 4
-          - 6
-          - 8
-          - 10
-          - 12
-          - 14
-          - 16
           - 18
+          - 20
+          - 22
         os:
           - ubuntu-latest
           - macos-latest
@@ -28,7 +23,4 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
       - run: npm install
-      - run: npm run test-without-lint
-      - name: run lint (node >= 6)
-        run: npm run lint
-        if: matrix.node-version >= 6
+      - run: npm test