change tests, fix compat with 1.6

[wheeheee]

• change to testing latest stable Julia version on windows-latest, macos-latest, ubuntu-latest
• 1.6 and nightly on ubuntu-latest
• simple_dot for 1.6 compat because of old BLAS bug

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (85fd99f) 92.23% compared to head (767f1c9) 92.33%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #40 +/- ## ========================================== + Coverage 92.23% 92.33% +0.10% ========================================== Files 5 5 Lines 502 496 -6 ========================================== - Hits 463 458 -5 + Misses 39 38 -1 ```

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[coveralls]

Pull Request Test Coverage Report for Build 7097375215

• 37 of 37 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 92.525%

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Totals
Change from base Build 6930122664:	0.3%
Covered Lines:	458
Relevant Lines:	495

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💛 - Coveralls