Remove support for Julia <1.6

[sostock]

The tests that are added in #551 reveal that some things don’t work correctly on Julia 1.0.

Julia 1.6 is the LTS release (since the release of Julia 1.7.0 on 2021-11-30), so support for older versions can be removed.

[codecov-commenter]

Codecov Report

Merging #553 (05d4b2a) into master (adf24e7) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #553   +/-   ##
=======================================
  Coverage   88.19%   88.20%           
=======================================
  Files          16       16           
  Lines        1449     1441    -8     
=======================================
- Hits         1278     1271    -7     
+ Misses        171      170    -1     

Impacted Files	Coverage Δ
src/quantities.jl	95.00% <0.00%> (ø)
src/user.jl	95.60% <0.00%> (+0.34%)	⬆️

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[giordano]

Should we make a release before merging this? There are some changes since last version: v1.11.0...adf24e7

[sostock]

I am slightly in favor of releasing a new version before merging this, because then people on Julia 1.0 still get #530 (which one could call a bugfix). The other changes since v1.11.0 are feature additions, so IMO it doesn’t make a difference whether they are released for Julia 1.0 or not. What do you think about this?

[giordano]

Sounds good to me. I just noticed that in #533 I put some if isdefined fences instead of if VERSION, those should all be true for Julia >= v1.6, so the fences can be safely removed in this PR

[sostock]

I just noticed that in #533 I put some if isdefined fences instead of if VERSION, those should all be true for Julia >= v1.6, so the fences can be safely removed in this PR

I just removed them.