chore: use towncrier for CHANGELOG

[SteveLauC]

What does this PR do

This PR adopts the CHANGELOG mode used by saltstack/salt to make our change log less conflict-prone

Still WIP, will add a document here on how it fits our release workflow

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

Looks like a good start. How does one execute the towncrier?

[SteveLauC]

Doc on how it fits into our workflow

For maintainers

How it works

The basic idea here is:

1. every PR would create a new file as its change log under the changelog
   directory
2. When we are going to create a new release, concatenate the change logs
   under the changelog directory, and prepend it to CHANGELOG.md

Automated concatenation and prepending

The step 2 of the last section will be automated with tool
towncrier

1. To install towncrier

   $ pip install towncrier

2. Basic usage

   Since nix is not a Python project, towncrier needs us to explictly tell it
   the version number of the next release.

   $ cd nix
   $ towncrier build --version=<VERSION>
   

   The best way to try this would be testing it in my fork:

   $ git clone https://github.com/SteveLauC/nix.git
   $ cd nix
   $ git checkout towncrier

   // CHANGELOG.md, before involving `towncrier`
   
   All notable changes to this project will be documented in this file.
   This project adheres to [Semantic Versioning](https://semver.org/).
   
   # Change Log
   
   
   ## [0.27.1] - 2023-08-28
   

   $ ls changelog
   1946.fixed.md  2103.added.md    2119.fixed.md    2136.changed.md  2139.changed.md  2142.added.md
   2074.added.md  2119.changed.md  2134.changed.md  2137.changed.md  2141.fixed.md    2144.changed.md
   
   # Here, I assume our next release would be 0.28.0
   $ towncrier build --version 0.28.0
   
   Loading template...
   Finding news fragments...
   Rendering news fragments...
   Writing to newsfile...
   Staging newsfile...
   I want to remove the following files:
   /tmp/nix/changelog/2144.changed.md
   /tmp/nix/changelog/2142.added.md
   /tmp/nix/changelog/2141.fixed.md
   /tmp/nix/changelog/2139.changed.md
   /tmp/nix/changelog/2137.changed.md
   /tmp/nix/changelog/2136.changed.md
   /tmp/nix/changelog/2134.changed.md
   /tmp/nix/changelog/2119.fixed.md
   /tmp/nix/changelog/2119.changed.md
   /tmp/nix/changelog/2103.added.md
   /tmp/nix/changelog/2074.added.md
   /tmp/nix/changelog/1946.fixed.md
   Is it okay if I remove those files? [Y/n]: Y
   Removing news fragments...
   Done!

   You can see that it is asking me if it is ok to remove the old files,
   and yes, it is fine and exactly what we want.

   towncrier build --version=<VERSION> --yes would assume this yes
   answer and is thus suitable for scripts.

   Let's check out our CHANGELOG.md, change log for 0.28.0 has been added!

   All notable changes to this project will be documented in this file.
   This project adheres to [Semantic Versioning](https://semver.org/).
   
   # Change Log
   
   ## [0.28.0] - 2023-10-04
   
   
   ### Added
   
   - Added `Ipv6HopLimit` to `::nix::sys::socket::ControlMessage` for Linux,
     MacOS, FreeBSD, DragonflyBSD, Android, iOS and Haiku.
     ([#2074](https://github.com/nix-rust/nix/pull/2074))
   - ...
   
   ### Changed
   
   - ...
   
   ### Fixed
   
   - ...
   
   ## [0.27.1] - 2023-08-28
   

   and the changelog directory has been cleared:

   $ ls -a changelog
   .  ..  .keep
   

When to involve towncrier

Probably we should do it in the PR that bumps our version in the
Cargo.toml

Retire cargo-release for altering CHANGELOG.md version and date

Before this, we are using cargo-release to replacing

[Unreleased] - ReleaseDate

with a specific version and date

We no longer need this with towncrier, it will do this for us as seen in the
above demo.

For contributors

For how to add a change log for a PR, see CONTRIBUTING.md.

[asomers]

Looks good! But ironically, you have a merge conflict in CHANGELOG.md.

[SteveLauC]

Merged this PR directly into master without all the CI checks since there are no code changes