[BACKPORT v4] Can't update any rancher2_role_template due to new external_rules attribute

[matttrach]

tracking #1396 as backport of #1389 addressing #1381

[k0da]

We desperately need that to be released, it blocks our delivery pipeline.

[joesims22]

Validated on v2.8-head id 68e5d43.

#	Priority	Description & Link	PASS/FAIL
1	P0	Verify updating RT via TFP is successful	✅ PASS
2	P0	Verify TFP sets external attribute to false by default	✅ PASS
3	P0	Verify external rules value is set to nil if external attribute is not set	✅ PASS
4	P0	Verify external rules cannot be set when updating RT via TFP while external is set to false	✅ PASS
5	P0	Verify external rules can be set when updating RT via TFP after setting external to true	✅ PASS
6	P1	Verify external rules cannot be set via kubectl when external is set to false	✅ PASS
7	P1	Verify external rules can be set when updating RT via kubectl after setting external to true	✅ PASS
8	P0	Verify external rules are removed when external set to false via TFP	✅ PASS
9	P0	Upgrade: Verify updating RT via TFP is successful upon upgrading Rancher	✅ PASS