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>

consensus-types/forkchoice/types.go

@@ -1,6 +1,8 @@
 package forkchoice
 
 import (
+	"time"
+
 	"github.com/OffchainLabs/prysm/v6/consensus-types/primitives"
 	eth "github.com/OffchainLabs/prysm/v6/proto/prysm/v1alpha1"
 )
@@ -47,7 +49,7 @@ type Node struct {
 	UnrealizedFinalizedEpoch primitives.Epoch
 	Balance                  uint64
 	Weight                   uint64
-	Timestamp                uint64
+	Timestamp                time.Time
 	BlockRoot                []byte
 	ParentRoot               []byte
 	ExecutionBlockHash       []byte