Use proper keyspace when updating the query graph of a reference DML

[frouioui]

Description

This PR fixes a regression introduced by #15107, when a user is using (use ks) a sharded keyspace that has a reference table with its source in an unsharded keyspace, the query planner would not set the target keyspace correctly when rewriting the query graph, leading to the following plan:

        {
          "QueryType": "UPDATE",
          "Original": "update ambiguous_ref_with_source set done = true where id = 1;",
          "Instructions": {
            "OperatorType": "Update",
            "Variant": "Reference",
            "Keyspace": {
              "Name": "user",
              "Sharded": true
            },
            "TargetTabletType": "PRIMARY",
            "Query": "update ambiguous_ref_with_source set done = true where id = 1",
            "Table": "ambiguous_ref_with_source"
          },
          "TablesUsed": [
            "main.ambiguous_ref_with_source",
            "user.ambiguous_ref_with_source"
          ]
        }

This plan ultimately leads to a failure in the engine as we cannot execute updates with a reference OpCode:

vitess/go/vt/vtgate/engine/update.go

Lines 65 to 73 in e3d2e89

	switch upd.Opcode {
	case Unsharded:
	return upd.execUnsharded(ctx, upd, vcursor, bindVars, rss)
	case Equal, EqualUnique, IN, Scatter, ByDestination, SubShard, MultiEqual:
	return upd.execMultiDestination(ctx, upd, vcursor, bindVars, rss, upd.updateVindexEntries, bvs)
	default:
	// Unreachable.
	return nil, fmt.Errorf("unsupported opcode: %v", upd.Opcode)
	}

This PR makes sure we are setting the target keyspace correctly.

Must be backported to 20 and 21.

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.41%. Comparing base (f6067e0) to head (579e92c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17226      +/-   ##
==========================================
+ Coverage   67.40%   67.41%   +0.01%     
==========================================
  Files        1570     1570              
  Lines      252903   252903              
==========================================
+ Hits       170460   170494      +34     
+ Misses      82443    82409      -34     

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[systay]

We should add an end-to-end test that asserts that this works well