Backport of docs: simplify Envoy version support docs into release/1.18.x #16921

Job	Run time
Get files changed and conditionally skip CI	27s
noop	1s
get-go-version / Determine Go toolchain version	6s
Setup	0s
check-go-mod	0s
lint-container-test-deps	0s
lint-consul-retry	0s
check-codegen	0s
lint-enums	0s
check-generated-protobuf	0s
lint	0s
dev-build	0s
lint-32bit	0s
go-test-enterprise	0s
go-test-race	0s
go-test-ce	0s
go-test-32bit	0s
go-test-api-${{ needs.get-go-version.outputs.go-version-previous }}	0s
go-test-envoyextensions	0s
go-test-troubleshoot	0s
go-test-api	0s
go-test-sdk	0s
go-test-sdk-${{ needs.get-go-version.outputs.go-version-previous }}	0s
go-tests-success	0s
	34s

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