Change process for CHANGELOG to news files

[rkm]

Proposed Changes

Changes the process for documenting changes in PRs to one where separate "news" files are created, then merged into the main CHANGELOG during the release process. This should hopefully prevent any merge conflicts when there are multiple PRs open at once.

This is performed via an automated script which collects the news files and generates something like:

## [2.0.0] 2021-03-02

## Feature

-   [#618](https://github.com/SMI/SmiServices/pull/618) by tznind.
    IsIdentifiableReviewer
    -   Added progress when loading large files (with cancellation support)
    -   Now groups outstanding failures by column
    -   Fixed rules being flagged as 'Identical' when classifying different
        input columns

## Bugfix

-   [#597](https://github.com/SMI/SmiServices/pull/597) by tznind. Fixed
    ConsensusRules not being run
-   [#619](https://github.com/SMI/SmiServices/pull/619) by jas88. Reduce memory
    consumption in nerd

## Meta

-   [#588](https://github.com/SMI/SmiServices/pull/588) by rkm. Prevent
    additional language packs being included in published packages. Reduces
    overall package size a bit.
-   [#616](https://github.com/SMI/SmiServices/pull/616) by rkm. Check for
    clobbered files during package build
-   [#620](https://github.com/SMI/SmiServices/pull/620) by rkm. Replace the
    legacy SecurityCodeScan with SecurityCodeScan.VS2019

Note that the diff to the CHANGELOG here is a one-time only thing in order to do an initial auto-format, and doesn't actually contain any additions.

Types of changes

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update (if none of the other choices apply)
  • In this case, ensure that the message of the head commit from the source branch is prefixed with [skip ci]

Checklist

By opening this PR, I confirm that I have:

• Reviewed the contributing guidelines for this repository
• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Updated any relevant API documentation
• Created or updated any tests if relevant
• Accurately updated the CHANGELOG
  • NOTE: This must include any changes to any of the following files: default.yaml, any of the RabbitMQ server configurations, GlobalOptions.cs
• Listed myself in the CONTRIBUTORS file 🚀
• Requested a review by one of the repository maintainers

Issues