Note the important internal change from argparse to click in v1.5 (#5660)

[Preview](https://docs-getdbt-com-git-dbeatty10-patch-2-dbt-labs.vercel.app/docs/dbt-versions/core-upgrade/upgrading-to-v1.5#behavior-changes)

## What are you changing in this pull request and why?

In v1.5, we changed the CLI internals from [`argparse` to
`click`](dbt-labs/dbt-core#8123 (comment)).
Since this marked an important change that affected some behavior, it
would be helpful to document that this change took place in v1.5.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.

---------

Co-authored-by: Matt Shaver <[email protected]>

Author: dbeatty10

website/docs/docs/dbt-versions/core-upgrade/05-upgrading-to-v1.5.md

@@ -24,7 +24,7 @@ dbt Labs is committed to providing backward compatibility for all versions 1.x,
 
 :::info Why changes to previous behavior?
 
-This release includes significant new features, and rework to `dbt-core`'s CLI and initialization flow. As part of refactoring its internals, we made a handful of changes to runtime configuration. The net result of these changes is more consistent & practical configuration options, and a more legible codebase.
+This release includes significant new features, and rework to `dbt-core`'s CLI and initialization flow. As part of refactoring its internals from [`argparse`](https://docs.python.org/3/library/argparse.html) to [`click`](https://click.palletsprojects.com), we made a handful of changes to runtime configuration. The net result of these changes is more consistent and practical configuration options, and a more legible codebase.
 
 **_Wherever possible, we will provide backward compatibility and deprecation warnings for at least one minor version before actually removing the old functionality._** In those cases, we still reserve the right to fully remove backwards compatibility for deprecated functionality in a future v1.x minor version of `dbt-core`.