chore: Replace fmt.Printf calls with warning logs

[wwared]

Description

As far as I can tell, there's only two places (outside of tests) that still are using fmt.Printf/fmt.Println to print warnings to stdout, in the frontend/schema directory. This small PR replaces them with warning logs using the logger module.

If the severity of the message should be different from a warning, let me know so I can change it quickly.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

I've tested this locally against a test case that triggers this message (specifically https://github.com/argumentcomputer/sphinx doing a Plonk BN254 proof verification, via an FFI wrapper at https://github.com/argumentcomputer/lurk-hs), and with this patch the messages are now logged at the proper level, as follows:

Before:

Testing fixture: "/home/w/dev/w/lurk-hs/assets/epoch_change.json"
ignoring uninitialized slice: Vars []frontend.Variable
ignoring uninitialized slice: Vars []frontend.Variable
ignoring uninitialized slice: Vars []frontend.Variable
Test completed for fixture: "/home/w/dev/w/lurk-hs/assets/epoch_change.json"

After:

Testing fixture: "/home/w/dev/w/lurk-hs/assets/epoch_change.json"
17:15:32 WRN ignoring uninitialized slice slice name=Vars slice type=[]frontend.Variable
17:15:32 WRN ignoring uninitialized slice slice name=Vars slice type=[]frontend.Variable
17:15:32 WRN ignoring uninitialized slice slice name=Vars slice type=[]frontend.Variable
Test completed for fixture: "/home/w/dev/w/lurk-hs/assets/epoch_change.json"

How has this been benchmarked?

This change is minor and has not been benchmarked.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[CLAassistant]

All committers have signed the CLA.