Update Julia version Compat

Increase lowest supported Julia version to 1.6
Set Compatibility to ^1.6 so anything <2 works
Update the CI to use Julia 1.6 instead of 1.4

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.4'  # Min Julia version supported
+          - '1.6'  # Min Julia version supported
           - '1'    # Expands to latest version
         os:
           - ubuntu-latest

Project.toml

@@ -23,4 +23,4 @@ ImageFiltering = "0.4 - 0.7"
 Images = "0.20 - 0.25"
 Interpolations = "0.10 - 0.14"
 PyPlot = "2.5 - 2.11"
-julia = "1.4"
+julia = "^1.6"