Remove v1.0 / v1.1 serialization format code. Require 3.6.0+ (which has newest serialization format) via increased Dart SDK constraint #868

Job	Run time
build (ubuntu, stable, native_assets_builder, false)	39s
build (ubuntu, stable, native_assets_cli, false)	37s
build (ubuntu, stable, native_toolchain_c, false)	38s
build (ubuntu, dev, native_assets_builder, false)	3m 17s
build (ubuntu, dev, native_assets_cli, false)	2m 9s
build (ubuntu, dev, native_toolchain_c, false)	2m 20s
build (macos, stable, native_assets_builder, false)	23s
build (macos, stable, native_assets_cli, false)	17s
build (macos, stable, native_toolchain_c, false)	23s
build (macos, dev, native_assets_builder, false)	3m 33s
build (macos, dev, native_assets_cli, false)	2m 10s
build (macos, dev, native_toolchain_c, false)	2m 45s
build (windows, stable, native_assets_builder, false)	1m 21s
build (windows, stable, native_assets_cli, false)	1m 42s
build (windows, stable, native_toolchain_c, false)	2m 8s
build (windows, dev, native_assets_builder, false)	4m 55s
build (windows, dev, native_assets_cli, false)	2m 34s
build (windows, dev, native_toolchain_c, false)	3m 58s
coverage-finished	0s
	35m 49s

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