☂ Migrate remaining configuration files to YAML format

## Summary

This umbrella issue tracks the migration of all remaining `.config` configuration files to YAML format. ATS has been progressively moving to YAML for configuration files to provide a more structured, readable, and maintainable configuration experience.

## Background

Several configuration files have already been converted to YAML:
- ✅ `ip_allow.yaml`
- ✅ `jsonrpc.yaml`
- ✅ `logging.yaml`
- ✅ `records.yaml`
- ✅ `sni.yaml`
- ✅ `strategies.yaml`

## Remaining Configuration Files

The following files still use the legacy `.config` format and should be migrated to YAML:

- [ ] `cache.config` → `cache.yaml` @bneradt (see #13456)
- [ ] `hosting.config` → `hosting.yaml`
- [x] ~~`parent.config` → `parent.yaml`~~
- [x] `plugin.config` → `plugin.yaml` @brbzull0 (see https://github.com/apache/trafficserver/pull/13070)
- [x] `remap.config` → `remap.yaml` @serrislew (see ##12997)
- [ ] `socks.config` → `socks.yaml`
- [ ] `splitdns.config` → `splitdns.yaml`
- [x] `ssl_multicert.config` → `ssl_multicert.yaml` @bneradt (see #12755)
- [x] `storage.config` → `storage.yaml` @masaori335 (see #13031)
- [x] `volume.config` → `volume.yaml` @masaori335 (see #13031)

## Related Issues

- #5590 - ☂ Migration to remap.yaml
- #7701 - When rewriting cache.config in YAML, make it not additive
- #10766 - Change `volume.config` to `volume.yaml`
- #5363 - ☂ Remove various _filename records.config variables

## Migration Guidelines

For each configuration file migration:
1. Design a YAML schema with a consistent top-level namespace (per #5245)
2. Implement the YAML parser alongside the existing parser
3. Provide a migration script or tool for users
4. Update documentation
5. Deprecate the old format with appropriate warnings
6. Remove the old format in a future major release

**Note**: these conversions should be done on the 11-Dev branch since these conversions will not be done in the 10.x release series.