By Jonathan Landay NEAR KHERSON FRONTLINE, Ukraine (Reuters) -A senior Ukrainian official predicted “the heaviest of battles” to come for the partially Russian occupied strategic southern province of Kherson and said Moscow’s military is digging in to face advancing Ukrainian forces. The region’s capital city and river port Kherson, which had a pre-war population of about 280,000, is the largest urban centre Russia still holds since capturing it early in the invasion of Ukraine eight months ago. Ukrainian forces do not appear to have gained much ground in their counter-offensive in Kherson sin…