Bump version to 8.1.8 (#5928) #3863

Job	Run time
ruby-3.2 driver-current mongodb-6.0 replica_set	12m 59s
ruby-3.1 driver-current mongodb-6.0 replica_set	13m 5s
ruby-3.0 driver-current mongodb-6.0 replica_set	13m 1s
ruby-3.1 driver-master mongodb-6.0 replica_set	12m 56s
ruby-3.0 driver-stable mongodb-6.0 replica_set	12m 54s
ruby-3.3 driver-current mongodb-7.0 server	11m 10s
ruby-3.2 driver-current mongodb-7.0 server	10m 53s
ruby-3.2 driver-current mongodb-7.0 server	10m 56s
ruby-3.1 driver-current mongodb-7.0 server	12m 24s
ruby-3.1 driver-current mongodb-6.0 server	11m 4s
ruby-3.0 driver-current mongodb-6.0 server	11m 6s
ruby-3.0 driver-current mongodb-6.0 server	11m 3s
ruby-2.7 driver-current mongodb-6.0 server	10m 52s
jruby-9.3 driver-current mongodb-6.0 server	18m 7s
ruby-3.1 driver-current mongodb-5.0 replica_set	12m 56s
ruby-2.7 driver-current mongodb-4.4 replica_set	12m 58s
	3h 18m 24s

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