Update node versions in CI environments (now 14-18)

Author: steveukx

Diff for: .changeset/grand-groovy-gargoyle.md

@@ -0,0 +1,5 @@
+---
+"simple-git": patch
+---
+
+Update CI environments to run build and test in node v18, drop node v12 now out of life.

Diff for: .github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       runs-on: ubuntu-latest
       strategy:
          matrix:
-            node-version: [12.x, 14.x, 16.x, 17.x]
+            node-version: [14.x, 16.x, 18.x]
       steps:
          - uses: actions/checkout@v2
          - name: Use Node.js ${{ matrix.node-version }}