docs: point TROUBLESHOOTING at the maintained mapstructure fork

[ump45nose]

Overview:

TROUBLESHOOTING.md still tells users that Viper unmarshals through github.com/mitchellh/mapstructure. That repository is archived; Viper replaced it with the fork it maintains, github.com/go-viper/mapstructure/v2 (see UPGRADE.md and PR #1723). This one-line docs change points the reference at the maintained package.

What problem does it solve?:

The troubleshooting page is the first thing users hit when Unmarshal misbehaves, and it sends them to dead documentation for the exact tag semantics Viper depends on. viper.go:40 imports github.com/go-viper/mapstructure/v2 and go.mod requires it directly, so the docs are the only place the old path survives (aside from UPGRADE.md, where it is intentionally discussed as the old dependency).

Special notes for a reviewer:

Docs-only; no code touched. Verified locally:

• go build ./... — clean
• go vet ./... — clean
• go test -count=1 ./... — all packages pass (viper, internal/encoding/{dotenv,json,toml,yaml})