feat: add REMOVE_OLD_CONFIGS to clear old configs before modpack install

[alberto-is]

Closes #3883

Adds a REMOVE_OLD_CONFIGS env var that clears the /data/config directory on server boot, before the modpack config is downloaded and unpacked. This is useful when updating a modpack, as old locally customized config files can otherwise be left behind and override the new config.

Changes

• Add REMOVE_OLD_CONFIGS env var (defaults to false), with CONFIG_OUT_DIR, REMOVE_OLD_CONFIGS_INCLUDE, REMOVE_OLD_CONFIGS_EXCLUDE and REMOVE_OLD_CONFIGS_DEPTH for tuning
• Run the cleanup at the start of start-setupModpack, before processing the modpack config
• Add removeOldConfigs to start-utils, following the existing removeOldMods implementation
• Document the new variable in docs/mods-and-plugins/index.md
• Add a setup-only test covering removal of an old config and installation of the pack config

Testing

• bash -n scripts/start-setupModpack scripts/start-utils
• tests/setuponlytests/remove-old-configs passes (remove-old-configs PASSED verify)