MINOR: lint: remove HAProxy 2.1 from e2e tests due to go 1.18 in the #523

Job	Run time
generate-specification	43s
HAProxy check commit message	0s
generate-models	1m 20s
lint-spec	6s
go mod tidy	12s
Build	18s
golangci-lint	1m 0s
Tests	25s
e2e on HAProxy (2.1)	29s
e2e on HAProxy (2.2)	33s
e2e on HAProxy (2.3)	33s
e2e on HAProxy (2.4)	34s
e2e on HAProxy (2.5)	31s
e2e on HAProxy (2.6)	32s
e2e on HAProxy (2.7)	30s
	7m 46s

Provide feedback