Upgrade react-intl to 6.8.9 to drop deprecated intl-messageformat-parser/intl-relativeformat
Removes the obsolete addLocaleData locale-data loading (dropped from react-intl since v3, native Intl support is now standard) and adds IntlLegacyBridge to republish the new context.intl.formatMessage() lookups working unchanged in perun-core and dependent projects.