Bump minimal supported julia version to 1.6

In #1900,
dependency on Roots.jl is being added, which only supports julia 1.6+

.github/workflows/CI.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.3'
+          - '1.6'
           - '1'
           - pre
         os:
@@ -36,7 +36,7 @@ jobs:
         with:
           version: ${{ matrix.version }}
           # ARM64 on macos-latest is neither supported by older Julia versions nor setup-julia
-          arch: ${{ matrix.os == 'macos-latest' && matrix.version != '1.3' && 'aarch64' || 'x64' }}
+          arch: ${{ matrix.os == 'macos-latest' && matrix.version >= '1.8' && 'aarch64' || 'x64' }}
           show-versioninfo: true
       - uses: julia-actions/cache@v2
       - uses: julia-actions/julia-buildpkg@v1

Project.toml

@@ -57,7 +57,7 @@ StatsAPI = "1.6"
 StatsBase = "0.32, 0.33, 0.34"
 StatsFuns = "0.9.15, 1"
 Test = "<0.0.1, 1"
-julia = "1.3"
+julia = "1.6"
 
 [extras]
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"