Don't test older Go versions

Support for wrapping errors was only introduced in 1.13, so don't bother
testing earlier versions. Also remove readme's promise of support on
earlier versions.

Author: rkennedy

.github/workflows/ci.yml

@@ -19,8 +19,6 @@ jobs:
           - 1.15.x
           - 1.14.x
           - 1.13.x
-          - 1.12.x
-          - 1.11.x
     steps:
     - uses: actions/checkout@v3
     - name: Set up Go ${{ matrix.go-version }}

README.md

@@ -10,8 +10,8 @@ and Mage automatically uses them as Makefile-like runnable targets.
 
 ## Installation
 
-Mage has no dependencies outside the Go standard library, and builds with Go 1.7
-and above (possibly even lower versions, but they're not regularly tested).
+Mage has no dependencies outside the Go standard library, and builds with Go 1.13
+and above.
 
 **Using GOPATH**