Fix bash 3.x compatibility issues in obolup script

[bussyjd]

Summary

• Fixed compatibility issues with macOS's default bash 3.2 by removing bash 4+ specific features
• Script now works on all macOS systems without requiring bash upgrade

Changes

1. Removed associative arrays (bash 4.0+ feature):

   • Replaced SUPPORTED_NETWORKS associative array with get_chain_id() function
   • Replaced SUPPORTED_MODES associative array with is_valid_mode() function
   • Used case statements for compatibility with bash 3.x

2. Removed nameref variables (bash 4.3+ feature):

   • Replaced declare -n with standard variable passing
   • Updated validation functions to return values via echo
   • Modified function calls to capture return values using command substitution

3. Fixed log output interference:

   • Redirected all log messages to stderr to prevent them from being captured in function return values
   • This ensures clean separation between logging and functional output

Testing

• Tested with bash 3.2.57 on macOS
• All functionality preserved while maintaining backward compatibility
• Script now works out-of-the-box on macOS without requiring bash upgrade

Context

macOS ships with bash 3.2 due to licensing (last GPL v2 version), so many users encounter issues with scripts using bash 4+ features. This fix ensures the obolup script works for all macOS users without additional setup.

[OisinKyne]

Looks fine, but could we merge my sepolia PR and then update this to also support sepolia before merging? :)