Set the minimum ecdsa version to v0.16.2

[DariuszDepta]

cosmwasm-crypto crate does not compile with -Zminimal-versions option because of improper dependency to ecdsa in k256 crate. To reproduce the problem, run:

cargo clean && rm Cargo.lock && cargo +nightly build -Zminimal-versions -p cosmwasm-crypto

The result will be:

   Compiling elliptic-curve v0.13.0
   Compiling ecdsa v0.16.0 <-----------------------------------------|
   Compiling k256 v0.13.1
   Compiling ed25519-zebra v3.0.0
   Compiling cosmwasm-crypto v1.4.0 (/home/confio/Work/CosmWasm/cosmwasm/packages/crypto)
error[E0599]: no function or associated item named `from_bytes` found for struct `ecdsa::Signature` in the current scope
  --> packages/crypto/src/secp256k1.rs:53:36
   |
53 |     let mut signature = Signature::from_bytes(&signature.into())
   |                                    ^^^^^^^^^^
   |                                    |
   |                                    function or associated item not found in `Signature<Secp256k1>`
   |                                    help: there is a method with a similar name: `to_bytes`

error[E0599]: no function or associated item named `from_bytes` found for struct `ecdsa::Signature` in the current scope
   --> packages/crypto/src/secp256k1.rs:115:32
    |
115 |     let signature = Signature::from_bytes(&signature.into())
    |                                ^^^^^^^^^^
    |                                |
    |                                function or associated item not found in `Signature<Secp256k1>`
    |                                help: there is a method with a similar name: `to_bytes`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `cosmwasm-crypto` (lib) due to 2 previous errors

The newest version of ecdsa crate is 0.16.8 but is not compatible with version 0.16.0. So specifying -Zminimal-versions uses 0.16.0 version and our code does not compile anymore. The minimum ecdsa version needed to compile cosmwams-crypto is 0.16.2.

This change adds minimum ecdsa dependendency set to 0.16.2.
Now the compilation is successful:

cargo clean && rm Cargo.lock && cargo +nightly build -Zminimal-versions -p cosmwasm-crypto

   Compiling serde_derive v1.0.103
   Compiling elliptic-curve v0.13.2
   Compiling ecdsa v0.16.2 <-----------------------------------------|
   Compiling k256 v0.13.1
   Compiling ed25519-zebra v3.0.0
   Compiling cosmwasm-crypto v1.4.0 (/home/confio/Work/CosmWasm/cosmwasm/packages/crypto)
    Finished dev [unoptimized + debuginfo] target(s) in 8.21s

I hope that after this change, all our crates that use cosmwasm-crypto directly or indirectly via cosmwasm-std will not have to fix ecdsa dependency in Cargo.tomls.