fix(ocpp2): use convertToDate() for all incoming date fields in handlers

fix(ocpp2): use convertToDate() for all incoming date fields in handlers

Replace raw new Date(string) and direct string comparisons with
convertToDate() for date fields in incoming OCPP 2.0 payloads:
- validateChargingProfile: validFrom/validTo comparisons
- validateChargingSchedule: startSchedule comparisons
- simulateFirmwareUpdateLifecycle: retrieveDateTime/installDateTime
- handleResponseHeartbeat: currentTime display
- OCPP20CertificateManager: cert.validFrom/validTo validation