Don't remove orphan peer if the peer count doesn't satisfy the replica requirement

[bufferflies]

Bug Report

The distribution of the tikv is defined as below:
instance label
tikv-1 zone:z1
tikv-2 zone:z1
tikv-3 zone:z1
tikv-4 zone:z2
tikv-5 zone:z2
tikv-6 zone:z2

AegionA located in [tikv-1,tikv-2,tikv3]
Then I build new rule to move regionA into the z2,
The first operator is remove orphan peer.

The unit test:

func (suite *ruleCheckerTestSuite) TestRemoveOrphanPeer() {
	suite.cluster.AddLabelsStore(1, 1, map[string]string{"zone": "z1", "host": "h1"})
	suite.cluster.AddLabelsStore(2, 1, map[string]string{"zone": "z1", "host": "h1"})
	suite.cluster.AddLabelsStore(3, 1, map[string]string{"zone": "z1", "host": "h1"})
	suite.cluster.AddLabelsStore(4, 1, map[string]string{"zone": "z2", "host": "h1"})
	suite.cluster.AddLabelsStore(5, 1, map[string]string{"zone": "z2", "host": "h2"})
	suite.cluster.AddLabelsStore(6, 1, map[string]string{"zone": "z2", "host": "h2"})
	rule := &placement.Rule{
		GroupID: "pd",
		ID:      "test2",
		Role:    placement.Voter,
		Count:   3,
		LabelConstraints: []placement.LabelConstraint{
			{
				Key:    "zone",
				Op:     placement.In,
				Values: []string{"z2"},
			},
		},
	}
	suite.ruleManager.SetRule(rule)
	suite.ruleManager.DeleteRule("pd", "default")

	suite.cluster.AddLeaderRegionWithRange(1, "200", "300", 1, 2, 3)
	op := suite.rc.Check(suite.cluster.GetRegion(1))
	suite.NotNil(op)
	suite.Equal("add-rule-peer", op.Desc())

	suite.cluster.AddLeaderRegionWithRange(2, "300", "400", 1, 2, 3, 4)
	op = suite.rc.Check(suite.cluster.GetRegion(2))
	suite.NotNil(op)
	suite.Equal("remove-orphan-peer", op.Desc())
}

What did you do?

What did you expect to see?

Add peer first and then remove orphan peer

What did you see instead?

What version of PD are you using (pd-server -V)?

6.5.4 master

[bufferflies]

related #6573

[seiya-annie]

/found customer