Reintroduce test on v1.6

Author: jishnub

.github/workflows/ci.yml

@@ -38,6 +38,7 @@ jobs:
       fail-fast: false
       matrix:
         version:
+          - '1.6'
           - '1'
           - '~1.10.0-0'
         os:

.github/workflows/downstream.yml

@@ -32,7 +32,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        julia-version: ['1']
+        julia-version: ['1', '1.6']
         os: [ubuntu-latest]
         package:
           - {repo: ApproxFun.jl, group: JuliaApproximation}