This changes the procedure in UPDATING to use etcupdate over megemaster.
This follows the recommendations in the handbook commited from D27848.
Commit message:
```
UPDATING: Change update procedure to use etcupdate(8) over mergemaster(8)
This commit aligns the steps in UPDATING with the steps from the handbook which already prefers etcupdate(8).
PR: 252417
Reviewed by: 0mp, ceri, imp
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D28062
````