Release `v4.0.0-alpha.1` (#1363)

* Add release notes for `v4.0.0-alpha.1`

* Mention breaking changes in changelog

* Bump version to `4.0.0-alpha.1`

* Fix header levels after merge

* Fix some formatting

* Add some notes about the breaking changes

* Add links to PRs

* Add note about linting move

* Bump `ink_linting` crate to `4.0.0-alpha.1`

* Remove `nightly` usage from delegator build script

* Use dashes instead of minus signs

Co-authored-by: Michael Müller <mich@elmueller.net>

* Beef up description of linter PR

Co-authored-by: Michael Müller <mich@elmueller.net>

* Move compatibility section to top of release notes

* Explicitly use `stable` channel

Our Docker images still default to `nightly`

* Build contracts with `stable` toolchain

* Install `rust-src` component before building examples

* Run the rest of the CI stages again

Co-authored-by: Michael Müller <mich@elmueller.net>