Unify changelog section order

This clarifies the order of subsections in the changelog. All entries
are now defined in the order `Packaging`, `Removed`, `Added`, `Changed`
and `Fixed`.

The sections `Security` and `Deprecated` have been removed. Since
deprecation should always just be a soft deprecation with clear user
warnings, it's not much different from just `Changed`, so that should
make it easier to decide where to put things. The `Security` section is
often not clear enough since a lot of changes might have some form of
security impact depending on interpretation. Since there's unlikely to
be any major security advisories from the Alacritty project, that
section is not particularly useful.

The `Added`, `Changed` and `Fixed` order follows the interest of the
user and the impact on them. This puts the most interesting new features
front and center while allowing to go for a deep dive to see if specific
bugs have been fixed.

The `Removed` section was put last since realistically this should not
affect users. All options should be gradually deprecated and no major
features should just get removed. So most of the time this will just
list deprecated features that are now completely removed, which usually
means there's no impact for users that updated from the last version.

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