Polish issues with mission difficulty dropdowns

[Smittytron]

• First commit fixes the TD dropdowns so that Normal isn't listed above Easy.
• Second First commit also standardizes TD default mission difficulties to Normal like we did with RA in Miscellaneous mission improvements 4 #19392.
• Last commit fixes the dropdown on the RA mission Down Under so that Normal appears selected.

[Smittytron]

Updated. Moved the two TD commits into one generic one to avoid rebase issues.

[dragunoff]

The changes here LGTM but there are a few RA missions that still have "easy" as the default.

Also, do we want to apply this to D2k as well? All of the missions there have "easy" as default.

[Smittytron]

Added the RA changes. D2K has an 'easy' difficulty in every mission unlike RA and TD. After raising the question in Discord I'd like to leave D2K as is for now.

[reaperrr]

Added the RA changes. D2K has an 'easy' difficulty in every mission unlike RA and TD. After raising the question in Discord I'd like to leave D2K as is for now.

Sounds reasonable to me.
Besides, at least judging by the missions I've played, iirc our D2k missions are notably harder than in the original pretty much across the board, so I think defaulting to 'Easy' is fine from that angle as well.
Things fluctuate quite a bit more in RA and TD due to the larger variety of people who ported the missions.

[abcdefg30]

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