Update gateway api version to 1.2.0 #8590

Job	Run time
Checks and variables	1m 21s
Unit Tests	47s
NJS Unit Tests	17s
Build Binary	32s
Build Plus images / build	58s
Build OSS images (nginx, linux/arm64, linux/amd64) / build	1m 17s
Build OSS images (ngf, linux/arm64, linux/amd64) / build	59s
Helm Tests	0s
Functional tests (nginx, v1.25.16) / Run Tests	13m 4s
Conformance tests (nginx, v1.25.16, true) / Run Tests	6m 48s
Conformance tests (nginx, v1.31.1, false) / Run Tests	6m 1s
Conformance tests (nginx, v1.25.16, false) / Run Tests	6m 25s
Conformance tests (nginx, v1.31.1, true) / Run Tests	6m 2s
Conformance tests (plus, v1.25.16, false) / Run Tests	6m 36s
Conformance tests (plus, v1.25.16, true) / Run Tests	6m 48s
Conformance tests (plus, v1.31.1, false) / Run Tests	6m 20s
Conformance tests (plus, v1.31.1, true) / Run Tests	6m 39s
Functional tests (nginx, v1.31.1) / Run Tests	13m 3s
Functional tests (plus, v1.25.16) / Run Tests	13m 50s
Functional tests (plus, v1.31.1) / Run Tests	11m 30s
Package and Publish Helm Chart	0s
	1h 49m 17s

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