Fix Address constructor and stale SDK version in EVM basics migration guide (#2690)

* Fix fabricated Address constructor and stale SDK version in EVM basics migration guide

### What
Replace the non-existent `Address::new(...)` call with the SDK's actual way to get an address in this context, and bump the pinned soroban-sdk version.

### Why
`Address` has no public byte-array constructor, so the example as written can't compile, and the pinned SDK version was a full major behind current.

* Use Address::from_str instead of a testutils-only constructor

### What
Construct the example address with `Address::from_str` instead of the test-only `Address::generate` helper.

### Why
This snippet is general reference code, not a test, so it shouldn't rely on an API that's only available under the `testutils` feature.

* Use a placeholder strkey instead of a fabricated real-looking address

### What
Show `"G..."` as the strkey argument instead of a made-up full-looking public key.

### Why
A real-looking G-address reads as if it were meaningful; a placeholder makes clear it's just illustrating the argument shape.

Author: leighmcculloch

docs/learn/migrate/evm/solidity-and-rust-basics.mdx

@@ -181,7 +181,7 @@ let vec = vec![&env, 0, 1, 2, 3];
 let map = map![&env, (2, 20), (1, 10)];
 
 // Address
-let address = Address::new([0u8; 32]);
+let address = Address::from_str(&env, "G...");
 
 // Struct (named fields)
 pub struct State {
@@ -573,10 +573,10 @@ crate-type = ["cdylib"]
 testutils = ["soroban-sdk/testutils"]
 
 [dependencies]
-soroban-sdk = "26"
+soroban-sdk = "27"
 
 [dev-dependencies]
-soroban-sdk = { version = "26", features = ["testutils"] }
+soroban-sdk = { version = "27", features = ["testutils"] }
 
 [profile.release]
 opt-level = "z"