Proc-scoped channels should not silently force codegen version 1.5

[mikex-oss]

Describe the bug

From #3679, when proc-scoped channels are enabled, codegen forces version 1.5.

This is still present at HEAD today:

xls/xls/tools/codegen_main.cc

Lines 76 to 78 in ca465af

	if (p->ChannelsAreProcScoped()) {
	codegen_flags_proto.set_codegen_version(CODEGEN_VERSION_ONE_DOT_FIVE);
	}

This behavior is unintuitive, since the user may be attempting to pin "codegen_version": "1.0" on a target, but XLS defaults proc-scoped channels to true, which overrides the user's choice.

xls/xls/dslx/ir_convert/convert_options.h

Line 69 in ca465af

bool lower_to_proc_scoped_channels = true;

Expected behavior
If proc-scoped channels are not compatible with codegen 1.0, it seems less surprising to error out.