fix: add changes for linter

[sylwang]

Proposed changes

• Modify .golangci.yml to suppress cyclops, depguard and varnamelen linters
• Suppress linters such as funlen and gocognit to avoid huge logic change
• use t.Log() for logging errors in testing to fix the forbidigo linter
• Update deprecated functions from io/ioutil
• Use utf8.DecodeRuneInString instead of utf8.DecodeRuneIn for the mirror linter
• Drop =0 for var declaration to fix the revive linter
• Remove leading spaces for nolintlint
• Remove unused linter

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING document
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works
• [x] If applicable, I have checked that any relevant tests pass after adding my changes
• [x] I have updated any relevant documentation (README.md)

[sylwang]

Once this is merged, will reopen this PR for fixing the config file parsing logic.