Restrict project_toml_formatting to julia 1.7 and later

[lgoettgens]

Resolves #208
Closes #206, closes #209.

Just make the test_toml_formatting a no-op on julia 1.6 and before.

[codecov]

Codecov Report

Merging #210 (2991b81) into master (19a5588) will decrease coverage by 12.83%.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master     #210       +/-   ##
===========================================
- Coverage   76.80%   63.97%   -12.83%     
===========================================
  Files          11       11               
  Lines         763      583      -180     
===========================================
- Hits          586      373      -213     
- Misses        177      210       +33     

Flag	Coverage Δ
unittests	63.97% <0.00%> (-12.83%)	⬇️

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Files	Coverage Δ
src/project_toml_formatting.jl	17.85% <0.00%> (-67.86%)	⬇️

... and 10 files with indirect coverage changes

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[gdalle]

Sorry I didn't mean to approve right away: the code changes are good but could you add tests, like the ones i tried to whip up in #206?

[lgoettgens]

Sorry I didn't mean to approve right away: the code changes are good but could you add tests, like the ones i tried to whip up in #206?

I would do that once the decision between this and #209 is made. I'll try to talk to @fingolfin this week about it.