Bump version to 8.1.8 #3858

Job	Run time
ruby-3.2 driver-current mongodb-6.0 replica_set	12m 36s
ruby-3.1 driver-current mongodb-6.0 replica_set	13m 20s
ruby-3.0 driver-current mongodb-6.0 replica_set	12m 39s
ruby-3.1 driver-master mongodb-6.0 replica_set	13m 21s
ruby-3.0 driver-stable mongodb-6.0 replica_set	12m 48s
ruby-3.3 driver-current mongodb-7.0 server	10m 58s
ruby-3.2 driver-current mongodb-7.0 server	10m 56s
ruby-3.2 driver-current mongodb-7.0 server	11m 38s
ruby-3.1 driver-current mongodb-7.0 server	10m 59s
ruby-3.1 driver-current mongodb-6.0 server	11m 6s
ruby-3.0 driver-current mongodb-6.0 server	11m 17s
ruby-3.0 driver-current mongodb-6.0 server	12m 2s
ruby-2.7 driver-current mongodb-6.0 server	10m 56s
jruby-9.3 driver-current mongodb-6.0 server	18m 41s
ruby-3.1 driver-current mongodb-5.0 replica_set	13m 18s
ruby-2.7 driver-current mongodb-4.4 replica_set	12m 56s
	3h 19m 31s

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