Update bitcoind dependency to 0.21.0 to fix M1 builds

same M1 build failure that (https://github.com/RCasatta/bitcoind/pull/38) solved is causing electrsd not to build on M1, which is causing bitcoindevkit build failures on my machine

Compiling electrsd v0.12.0 (/Users/futurepaul/dev/rust/other-peoples-code/electrsd)
error[E0425]: cannot find function `download_filename` in this scope
  --> /Users/futurepaul/.cargo/registry/src/github.com-1ecc6299db9ec823/bitcoind-0.20.0/build.rs:49:29
   |
49 |     let download_filename = download_filename();
   |                             ^^^^^^^^^^^^^^^^^ not found in this scope

error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> /Users/futurepaul/.cargo/registry/src/github.com-1ecc6299db9ec823/bitcoind-0.20.0/build.rs:62:9
    |
62  | /         println!(
63  | |             "filename:{} version:{} hash:{}",
64  | |             download_filename, VERSION, expected_hash
65  | |         );
    | |__________^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
note: required by a bound in `ArgumentV1::<'a>::new`
   --> /Users/futurepaul/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:293:20
    |
293 |     pub fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter<'_>) -> Result) -> ArgumentV1<'b> {
    |                    ^ required by this bound in `ArgumentV1::<'a>::new`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> /Users/futurepaul/.cargo/registry/src/github.com-1ecc6299db9ec823/bitcoind-0.20.0/build.rs:67:19
    |
67  |           let url = format!(
    |  ___________________^
68  | |             "https://bitcoincore.org/bin/bitcoin-core-{}/{}",
69  | |             VERSION, download_filename
70  | |         );
    | |_________^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
note: required by a bound in `ArgumentV1::<'a>::new`
   --> /Users/futurepaul/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:293:20
    |
293 |     pub fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter<'_>) -> Result) -> ArgumentV1<'b> {
    |                    ^ required by this bound in `ArgumentV1::<'a>::new`
    = note: this error originates in the macro `$crate::__export::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)

Some errors have detailed explanations: E0277, E0425.
For more information about an error, try `rustc --explain E0277`.

RCasatta / electrsd

Update bitcoind dependency to 0.21.0 to fix M1 builds #25