GH-45784: [C++] Unpin LLVM and OpenSSL in Brewfile (#45785)

### Rationale for this change

We don't need to pin LLVM and OpenSSL now.

### What changes are included in this PR?

Unpin LLVM and OpenSSL.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45784

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

cpp/Brewfile

@@ -29,12 +29,12 @@ brew "git"
 brew "glog"
 brew "googletest"
 brew "grpc"
-brew "llvm@14"
+brew "llvm"
 brew "lz4"
 brew "mimalloc"
 brew "ninja"
 brew "node"
-brew "openssl@3"
+brew "openssl"
 brew "pkgconf"
 brew "protobuf"
 brew "python"