Disable Aqua tests for julia 1.6 too

I *think* this is a false-positive:
```
Running tests:
1 ambiguities found. To get a list, set `broken = false`.
Ambiguity #1
frule(::ChainRulesCore.RuleConfig, args...) in ChainRulesCore at /home/runner/.julia/packages/ChainRulesCore/I1EbV/src/rules.jl:64
frule(::Any, ::typeof(Distributions.logpdf), d::Distributions.Uniform, x::Real) in Distributions.DistributionsChainRulesCoreExt at /home/runner/work/Distributions.jl/Distributions.jl/ext/DistributionsChainRulesCoreExt/univariate/continuous/uniform.jl:1

Possible fix, define
  frule(::ChainRulesCore.RuleConfig, ::typeof(Distributions.logpdf), ::Distributions.Uniform, ::Real)

Aqua: Test Failed at /home/runner/.julia/packages/Aqua/tHrmY/src/ambiguities.jl:78
  Expression: iszero(num_ambiguities)
Stacktrace:
 [1] _test_ambiguities(packages::Vector{Base.PkgId}; broken::Bool, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Aqua ~/.julia/packages/Aqua/tHrmY/src/ambiguities.jl:78
 [2] _test_ambiguities
   @ ~/.julia/packages/Aqua/tHrmY/src/ambiguities.jl:69 [inlined]
 [3] test_ambiguities(packages::Module; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Aqua ~/.julia/packages/Aqua/tHrmY/src/ambiguities.jl:28
 [4] test_ambiguities(packages::Module)
   @ Aqua ~/.julia/packages/Aqua/tHrmY/src/ambiguities.jl:28
 [5] macro expansion
   @ ~/work/Distributions.jl/Distributions.jl/test/aqua.jl:19 [inlined]
 [6] macro expansion
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
 [7] top-level scope
   @ ~/work/Distributions.jl/Distributions.jl/test/aqua.jl:9
```

test/aqua.jl

@@ -15,7 +15,7 @@ import Aqua
  )
  # Tests are not reliable on older Julia versions and
  # show ambiguities in loaded packages
- if VERSION >= v"1.6"
+ if VERSION >= v"1.7"
  Aqua.test_ambiguities(Distributions)
  end
 end