bor: milestone avoid TryLock (#12422)

`TryLock` - tells that mutex was locked - but doesn't tell by whom.
maybe by parent caller, maybe by another goroutine.

So next pattern is bad:
```
if m.finality.TryLock() {
   defer m.finality.Unlock()
}

// mutate
```
because mutex can be locked by another goroutine and our func will
mutate fields without waiting for mutex.

```
==================
WARNING: DATA RACE
Write at 0x00c004e64688 by goroutine 28583:
  github.com/erigontech/erigon/polygon/bor/finality/whitelist.(*milestone).UnlockSprint()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist/milestone.go:175 +0x106
  github.com/erigontech/erigon/polygon/bor/finality.fetchWhitelistMilestone()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist_helpers.go:113 +0xb3b
  github.com/erigontech/erigon/polygon/bor/finality.handleMilestone()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:202 +0xc8
  github.com/erigontech/erigon/polygon/bor/finality.retryHeimdallHandler()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:159 +0x5cb
  github.com/erigontech/erigon/polygon/bor/finality.RetryHeimdallHandler()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:117 +0x5c
  github.com/erigontech/erigon/polygon/bor/finality.startMilestoneWhitelistService()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:93 +0x2a
  github.com/erigontech/erigon/polygon/bor/finality.Whitelist.gowrap2()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:64 +0x17

Previous write at 0x00c004e64688 by goroutine 28584:
  github.com/erigontech/erigon/polygon/bor/finality/whitelist.(*milestone).RemoveMilestoneID()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist/milestone.go:194 +0x164
  github.com/erigontech/erigon/polygon/bor/finality.handleNoAckMilestone()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:237 +0x12c
  github.com/erigontech/erigon/polygon/bor/finality.retryHeimdallHandler()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:159 +0x5cb
  github.com/erigontech/erigon/polygon/bor/finality.RetryHeimdallHandler()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:117 +0x57
  github.com/erigontech/erigon/polygon/bor/finality.startNoAckMilestoneService()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:102 +0x2a
  github.com/erigontech/erigon/polygon/bor/finality.Whitelist.gowrap3()
      github.com/erigontech/erigon/polygon/bor/finality/whitelist.go:65 +0x17
```

polygon/bor/finality/whitelist/milestone.go

@@ -120,7 +120,7 @@ func (m *milestone) Process(block uint64, hash common.Hash) {
 
 	whitelistedMilestoneMeter.SetUint64(block)
 
-	m.UnlockSprint(block)
+	m.unlockSprint(block)
 }
 
 // LockMutex This function will Lock the mutex at the time of voting
@@ -145,7 +145,7 @@ func (m *milestone) UnlockMutex(doLock bool, milestoneId string, endBlockNum uin
 	m.Locked = m.Locked || doLock
 
 	if doLock {
-		m.UnlockSprint(m.LockedMilestoneNumber)
+		m.unlockSprint(m.LockedMilestoneNumber)
 		m.Locked = true
 		m.LockedMilestoneHash = endBlockHash
 		m.LockedMilestoneNumber = endBlockNum
@@ -168,8 +168,16 @@ func (m *milestone) UnlockSprint(endBlockNum uint64) {
 		return
 	}
 
-	if m.finality.TryLock() {
-		defer m.finality.Unlock()
+	m.finality.Lock()
+	defer m.finality.Unlock()
+
+	m.unlockSprint(endBlockNum)
+}
+
+// UnlockSprint This function will unlock the locked sprint
+func (m *milestone) unlockSprint(endBlockNum uint64) {
+	if endBlockNum < m.LockedMilestoneNumber {
+		return
 	}
 
 	m.Locked = false
@@ -233,12 +241,6 @@ func (m *milestone) GetMilestoneIDsList() []string {
 
 // This is remove the milestoneIDs stored in the list.
 func (m *milestone) purgeMilestoneIDsList() {
-	// try is used here as the finality lock is preserved over calls - so the lock state
-	// is not clearly defined in the local code - this likely needs to be revised
-	if m.finality.TryLock() {
-		defer m.finality.Unlock()
-	}
-
 	m.LockedMilestoneIDs = make(map[string]struct{})
 }