chore: use std.PrevRealm() in grc721 package (#992)

update (missing part of) grc721 package implementation to use
`std.PrevRealm()` not `std.GetOrigCaller()`

## related
#667 implementation of `std.PrevRealm()`
#895 update grc20, 721 to use `std.PrevRealm()`

<details><summary>Checklists...</summary>

## Contributors Checklist

- [x] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](../.benchmarks/README.md).
</details>

Co-authored-by: Manfred Touron <94029+moul@users.noreply.github.com>

examples/gno.land/p/demo/grc/grc721/basic_nft.gno

@@ -95,7 +95,7 @@ func (s *basicNFT) Approve(to std.Address, tid TokenID) error {
 		return ErrApprovalToCurrentOwner
 	}
 
-	caller := std.GetOrigCaller()
+	caller := std.PrevRealm().Addr()
 	if caller != owner && !s.IsApprovedForAll(owner, caller) {
 		return ErrCallerIsNotOwnerOrApproved
 	}
@@ -123,15 +123,15 @@ func (s *basicNFT) SetApprovalForAll(operator std.Address, approved bool) error
 		return ErrInvalidAddress
 	}
 
-	caller := std.GetOrigCaller()
+	caller := std.PrevRealm().Addr()
 	return s.setApprovalForAll(caller, operator, approved)
 }
 
 // Safely transfers `tokenId` token from `from` to `to`, checking that
 // contract recipients are aware of the GRC721 protocol to prevent
 // tokens from being forever locked.
 func (s *basicNFT) SafeTransferFrom(from, to std.Address, tid TokenID) error {
-	caller := std.GetOrigCaller()
+	caller := std.PrevRealm().Addr()
 	if !s.isApprovedOrOwner(caller, tid) {
 		return ErrCallerIsNotOwnerOrApproved
 	}
@@ -150,7 +150,7 @@ func (s *basicNFT) SafeTransferFrom(from, to std.Address, tid TokenID) error {
 
 // Transfers `tokenId` token from `from` to `to`.
 func (s *basicNFT) TransferFrom(from, to std.Address, tid TokenID) error {
-	caller := std.GetOrigCaller()
+	caller := std.PrevRealm().Addr()
 	if !s.isApprovedOrOwner(caller, tid) {
 		return ErrCallerIsNotOwnerOrApproved
 	}