Specify correct supported puppet version, 8.x only #66

Job	Run time
Released / Puppet8 gem on Ruby 3.1 on Linux	23s
Released / Puppet8 gem on Ruby 3.1 on macOS	31s
Released / Puppet8 gem on Ruby 3.1 on Windows	1m 39s
Nightly / Puppet8 gem on Ruby 3.1 on Windows	17m 0s
Nightly / Puppet8 gem on Ruby 3.1 on Linux	1m 44s
	21m 17s

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