cs/manifest: allow updating kafka start offset with the same offset

Previously validation was failing when `_kafka_start_offset` was
overridden with the same offset. Since re-setting the offset to the same
number doesn't change anything we may relax the validation here.

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit b1c1d01)

src/v/cloud_storage/partition_manifest.cc

@@ -730,7 +730,7 @@ void partition_manifest::set_archive_clean_offset(
 
 bool partition_manifest::advance_start_kafka_offset(
   kafka::offset new_start_offset) {
-    if (_start_kafka_offset_override >= new_start_offset) {
+    if (_start_kafka_offset_override > new_start_offset) {
         return false;
     }
     _start_kafka_offset_override = new_start_offset;

src/v/cloud_storage/tests/partition_manifest_test.cc

@@ -1431,9 +1431,9 @@ SEASTAR_THREAD_TEST_CASE(test_partition_manifest_start_kafka_offset_advance) {
       m.get_start_kafka_offset_override(), kafka::offset(370));
     BOOST_REQUIRE_EQUAL(m.get_start_offset(), model::offset(100));
     BOOST_REQUIRE_EQUAL(m.get_start_kafka_offset(), kafka::offset(90));
-
+    // Allow update with the same value
+    BOOST_REQUIRE(m.advance_start_kafka_offset(kafka::offset(370)));
     // If trying to move back, it should no-op.
-    BOOST_REQUIRE(!m.advance_start_kafka_offset(kafka::offset(370)));
     BOOST_REQUIRE(!m.advance_start_kafka_offset(kafka::offset(369)));
     BOOST_REQUIRE_EQUAL(
       m.get_start_kafka_offset_override(), kafka::offset(370));