Add Psalm 6 support, drop Psalm 4 support

This release adds support for Psalm 6, dropping support for Psalm 4.

What's Changed

• Update README.md by @sander-bol in #128
• Replace deprecated ::set-output by @weirdan in #132
• Dependabot updates for GH actions by @weirdan in #133
• Bump actions/checkout from 2 to 3 by @dependabot in #134
• Bump actions/cache from 2 to 3 by @dependabot in #135
• Switch to MixedReturnStatement, drop Psalm 4 support by @danog in #142

New Contributors

• @sander-bol made their first contribution in #128
• @dependabot made their first contribution in #134

Full Changelog: 0.18.4...0.19.0