Use time.Time instead of uint64 for genesis time (#15419)

* Convert genesis times from seconds to time.Time

* Fixing failed forkchoice tests in a new commit so it doesn't get worse

Fixing failed spectest tests in a new commit so it doesn't get worse

Fixing forkchoice tests, then spectests

* Fixing forkchoice tests, then spectests. Now asking for help...

* Fix TestForkChoice_GetProposerHead

* Fix broken build

* Resolve TODO(preston) items

* Changelog fragment

* Resolve TODO(preston) items again

* Resolve lint issues

* Use consistant field names for sinceSlotStart (no spaces)

* Manu's feedback

* Renamed StartTime -> UnsafeStartTime, marked as deprecated because it doesn't handle overflow scenarios.
Renamed SlotTime -> StartTime
Renamed SlotAt -> At
Handled the error in cases where StartTime was used.

@james-prysm feedback

* Revert beacon-chain/blockchain/receive_block_test.go from 1b7844de

* Fixing issues after rebase

* Accepted suggestions from @potuz

* Remove CanonicalHeadSlot from merge conflicts

---------

Co-authored-by: potuz <potuz@prysmaticlabs.com>

beacon-chain/execution/service.go

@@ -711,7 +711,7 @@ func (s *Service) cacheBlockHeaders(start, end uint64) error {
 // Determines the earliest voting block from which to start caching all our previous headers from.
 func (s *Service) determineEarliestVotingBlock(ctx context.Context, followBlock uint64) (uint64, error) {
 	genesisTime := s.chainStartData.GenesisTime
-	currSlot := slots.CurrentSlot(genesisTime)
+	currSlot := slots.CurrentSlot(time.Unix(int64(genesisTime), 0)) // lint:ignore uintcast -- Genesis time will never exceed int64 in seconds.
 
 	// In the event genesis has not occurred yet, we just request to go back follow_distance blocks.
 	if genesisTime == 0 || currSlot == 0 {
@@ -837,7 +837,7 @@ func (s *Service) validPowchainData(ctx context.Context) (*ethpb.ETH1ChainData,
 		}
 		s.chainStartData = &ethpb.ChainStartData{
 			Chainstarted:       true,
-			GenesisTime:        genState.GenesisTime(),
+			GenesisTime:        uint64(genState.GenesisTime().Unix()),
 			GenesisBlock:       0,
 			Eth1Data:           genState.Eth1Data(),
 			ChainstartDeposits: make([]*ethpb.Deposit, 0),