Remove v1.0 / v1.1 serialization format code. Require 3.6.0+ (which has newest serialization format) via increased Dart SDK constraint #876

Job	Run time
build (ubuntu, stable, native_assets_builder, false)	5m 28s
build (ubuntu, stable, native_assets_cli, false)	2m 0s
build (ubuntu, stable, native_toolchain_c, false)	2m 5s
build (ubuntu, dev, native_assets_builder, false)	5m 43s
build (ubuntu, dev, native_assets_cli, false)	2m 14s
build (ubuntu, dev, native_toolchain_c, false)	2m 7s
build (macos, stable, native_assets_builder, false)	6m 9s
build (macos, stable, native_assets_cli, false)	2m 48s
build (macos, stable, native_toolchain_c, false)	2m 25s
build (macos, dev, native_assets_builder, false)	5m 44s
build (macos, dev, native_assets_cli, false)	2m 14s
build (macos, dev, native_toolchain_c, false)	2m 38s
build (windows, stable, native_assets_builder, false)	7m 59s
build (windows, stable, native_assets_cli, false)	2m 42s
build (windows, stable, native_toolchain_c, false)	4m 3s
build (windows, dev, native_assets_builder, false)	7m 32s
build (windows, dev, native_assets_cli, false)	3m 2s
build (windows, dev, native_toolchain_c, false)	3m 31s
coverage-finished	3s
	1h 10m 27s

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