Replace the changelog regex parser with Parslet

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50063 we
introduced code for generating Markdown changelogs using the API, using
a custom template language. The compiler for this language would convert
a custom syntax into ERB, making sure arbitrary code execution isn't
possible; or so we thought. In
https://gitlab.com/gitlab-org/gitlab/-/issues/300224 we found a way to
bypass the template engine's harness, and run arbitrary Ruby code.

In response to this issue, I decided to investigate replacing the setup
with something more secure. We always planned on doing so when deemed
necessary, unfortunately that need arrived sooner than expected.

In this commit we replace the regex/ERB based setup with a parser built
using Parslet (http://kschiess.github.io/parslet/). Parslet makes it
pretty easy to write a parser, and was already an indirect dependency of
GitLab (through the license_finder Gem). This new parser doesn't allow
for arbitrary code execution, doesn't depend on ERB, and is less fragile
compared to the old setup. Templates are executed by walking and
evaluating the AST nodes the parser produces. While this won't break any
speed records, it's easy to maintain and understand, and fast enough for
our needs.

In this new setup there is a slight difference compared to the old
setup. In the old setup, expression tags on their own line don't add a
new line. So this:

    foo
    {% if something %}{% end %}
    bar

Compiles into this:

    foo
    bar

Getting this right using the Parslet parser proved difficult, so we took
a slightly different approach: any newline following an expression tag
(if, else, end, and each) consumes the newline that directly follows it
(if any). This requires only a small change in the template, is easy to
implement, and still intuitive to the user.

As part of this commit we also fix a small bug that would lead to empty
entries being included in the template, and add a note about using the
right YAML syntax to preserve newlines.