Bump Cargo.toml to WIP version (0.5.0+git)

We bump the version number on `main` to the next-planned minor release
(mod `+git` metadata tag). This aligns what we're planning to do in LDK
(see lightningdevkit/rust-lightning#3546) and
allows us to run SemVer checks on PRs towards `main`.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ldk-node"
-version = "0.4.3"
+version = "0.5.0+git"
 authors = ["Elias Rohrer <dev@tnull.de>"]
 homepage = "https://lightningdevkit.org/"
 license = "MIT OR Apache-2.0"