test: add msrv verify as ci check

[V0ldek]

Short description

The MSRV got unknowingly bumped before, with this CI check we will avoid it in the future.

This also fixes a violation of MSRV in rsonpath-syntax unknowingly introduced in #477

Checklist

All of these should be ticked off before you submit the PR.

• I ran just verify locally and it succeeded.
• Issue was given go ahead and is linked above OR I have included justification for a minor change.
• Unit tests for my changes are included OR no functionality was changed.