Replace whats_new template in HAML with Markdown

[tnir]

Replace whats_new template in HAML with one in Markdown.

• Counterpart PR for the existing what's new pages: Rewrite all existing what's new pages from HAML to Markdown #1051
• Updates Rake task to create new minor version #307

Why Markdown over HAML

We do not need rich but slow technology like HAML just for writing release notes, indeed. GitHub Releases feature shows it enough to us.

We will get

via rake versions:create:

# What's New in v2.5

The [Bundler 2.5 announcement](/blog/2023/01/02/bundler-v2-5.html)
includes context and a more detailed explanation of the changes in this version.
This is a summary of the biggest changes. As always, a detailed list of every change is provided in
[the changelog](https://github.com/rubygems/rubygems/blob/3.5/bundler/CHANGELOG.md).

## BIG NEW THING

THE BIG NEW THING DOES COOL NEW STUFF.

Bundler 2.5 also includes:

- SMALL NEW THING FIXES A PROBLEM

<a href="https://github.com/rubygems/rubygems/blob/3.5/bundler/CHANGELOG.md" class="btn btn-primary">Full 2.5 changelog</a>

Signed-off-by: Takuya Noguchi [email protected]

[tnir]

@indirect @deivid-rodriguez Any thoughts?

[deivid-rodriguez]

Nice improvement! 👍