psalm is always using PHP target version 5.6

[defuse]

I noticed this while running it locally:

$ vendor/bin/psalm
Warning: "findUnusedBaselineEntry" will default to "true" in Psalm 6. You should explicitly enable or disable this setting.
Warning: "findUnusedCode" will default to "true" in Psalm 6. You should explicitly enable or disable this setting.

Install the opcache extension to make use of JIT on PHP 8.0+ for a 20%+ performance boost!

Target PHP version: 5.6 (inferred from composer.json).
Scanning files...
Analyzing files..

This means our CI, which attempts to run psalm for different PHP versions, is actually just doing the same thing multiple times. We need to pass in --php-version=<the PHP version>.

Edit: Actually I think it may still be doing something different for the different versions of PHP, it's at least running psalm under that version of PHP. But we probably want to be testing with different target PHP versions, right?