Use slog (stdlib) as default logger, move logrus to subpackage

[paganotoni]

Summary

This PR refactors the logger to use Go's standard library log/slog as the default implementation instead of logrus. The logrus implementation is moved to a subpackage for users who still need it.

Changes

• Root package: Now uses log/slog (stdlib) with zero external dependencies
• logrus subpackage: Contains the original logrus implementation
• go.mod: Updated to require Go 1.25
• Code modernization: Use any instead of interface{}
• Documentation: Added package-level docs for both packages
• Makefile: Removed legacy GO111MODULE checks

Benefits

1. Smaller dependency tree: Users only depend on logrus if they explicitly import github.com/gobuffalo/logger/logrus
2. Modern Go practices: Uses stdlib slog instead of third-party library
3. Zero external dependencies for default usage
4. Module graph pruning: Go will not download logrus for users who don't import the subpackage

Migration Guide

For existing users (no changes needed)

import "github.com/gobuffalo/logger"
log := logger.New(logger.InfoLevel)

For users who need logrus

import "github.com/gobuffalo/logger/logrus"
log := logrus.New(logrus.InfoLevel)

Breaking Changes

The default logger is now slog-based. While the Logger and FieldLogger interfaces remain the same, there may be subtle behavioral differences:

• Log output format may differ slightly
• Field handling uses slog attributes internally

Users relying on logrus-specific behavior should import the logrus subpackage.