Update MSRV to 1.51, enable cargo resolver = "2"

This is required to keep using dependencies that have switched to
MSRV 1.51 which requires the resolver version 2: tiny-bip39, ahash, and object.

.github/workflows/cont_integration.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         rust:
           - 1.53.0 # STABLE
-          - 1.46.0 # MSRV
+          - 1.51.0 # MSRV
         features:
           - default
           - minimal

Cargo.toml

@@ -10,6 +10,7 @@ description = "A modern, lightweight, descriptor-based wallet library"
 keywords = ["bitcoin", "wallet", "descriptor", "psbt"]
 readme = "README.md"
 license = "MIT OR Apache-2.0"
+resolver = "2"
 
 [dependencies]
 bdk-macros = "^0.6"
@@ -24,7 +25,7 @@ rand = "^0.7"
 sled = { version = "0.34", optional = true }
 electrum-client = { version = "0.8", optional = true }
 rusqlite = { version = "0.25.3", optional = true }
-ahash = { version = "=0.7.4", optional = true }
+ahash = { version = "0.7", optional = true }
 reqwest = { version = "0.11", optional = true, features = ["json"] }
 ureq = { version = "2.1", features = ["json"], optional = true }
 futures = { version = "0.3", optional = true }
@@ -34,10 +35,7 @@ cc = { version = ">=1.0.64", optional = true }
 socks = { version = "0.3", optional = true }
 lazy_static = { version = "1.4", optional = true }
 
-# the latest 0.8 version of tiny-bip39 depends on zeroize_derive 1.2 which has MSRV 1.51 and our 
-# MSRV is 1.46, to fix this until we update our MSRV or replace the tiny-bip39 
-# dependency https://github.com/bitcoindevkit/bdk/issues/399 we can only use an older version
-tiny-bip39 = { version = "< 0.8", optional = true }
+tiny-bip39 = { version = "0.8", optional = true }
 
 bitcoinconsensus = { version = "0.19.0-3", optional = true }