Update GitHub workflow

[iarekk]

Several changes:

1. Add elixir 1.16.x / OTP 26.2 to the matrix.
2. Upgrade credo to 1.7.4.
   Reason: Credo 1.7.0 issues a huge number of warnings about itself when compiled with Elixir 1.6.x, update to 1.7.4 fixes it. Some discussion in the Credo repo here: Be explicit about steps to avoid warnings rrrene/credo#1085
3. Address the newly-appeared credo warnings, all of which had the following structure:

   ┃ [R] ↗ Predicate function names should not start with 'is', and should end in a question mark.
   ┃       lib/sobelow/utils.ex:10:7 #(Sobelow.Utils.is_router?)
   

   The change is to rename functions such as is_router? to router?

[iarekk]

Potentially this fixes #152. Happy to break down the PR into smaller chunks if needed!

[iarekk]

https://hexdocs.pm/credo/Credo.Check.Readability.PredicateFunctionNames.html and this is the new Credo check that was breaking the builds after upgrading to 1.7.4

[houllette]

Hey @iarekk - thank you so much for this QoL PR, really appreciate the assistance!

This is a pretty sizable update, so your dedication to fixing all the credo warnings is really awesome - normally I'd prefer to break this PR up a bit (one for the GitHub Action change, one for the credo bump), but I think it makes sense in this case plus the work is already done and all working together harmoniously / passing checks. So we're good to go here!

[iarekk]

Thank you @houllette! I'm very happy to help the project – and will definitely keep the PRs smaller nex time 😄