[ip6] expose static multicast address getters and remove wrapper methods (#12416)

This commit updates `Ip6::Address` to expose `static` getter methods for
common multicast addresses (e.g., `GetLinkLocalAllNodesMulticast()`)
as public API.

Consequentially, the wrapper `SetTo...` and `Is...` methods for these
multicast addresses are removed. Callers are updated to use the
`static` getters directly. This change simplifies usage by allowing
direct access to the constant address instances, often eliminating
the need for local `Ip6::Address` variables.

src/core/thread/tmf.cpp

@@ -58,7 +58,7 @@ void MessageInfo::SetSockAddrToRlocPeerAddrToLeaderRloc(void)
 void MessageInfo::SetSockAddrToRlocPeerAddrToRealmLocalAllRoutersMulticast(void)
 {
     SetSockAddrToRloc();
-    GetPeerAddr().SetToRealmLocalAllRoutersMulticast();
+    SetPeerAddr(Ip6::Address::GetRealmLocalAllRoutersMulticast());
 }
 
 void MessageInfo::SetSockAddrToRlocPeerAddrTo(uint16_t aRloc16)