Improvements for osu!mania Argon skin

[smoogipoo]

Here are some issues that were brought up to me in discussions at COE:

Issues

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1. It's impossible to play with colour blindness.
2. Hold note heads cast a shadow over the body, which is hard to read at high speeds.
3. Hold note bodies extend too far, making "stairs" involving the ends of hold notes hard to read (they extend beyond the point where a note in the adjacent column is supposed to be hit). Examples/possible solutions: https://github.com/ppy/osu/assets/1329837/b56786d1-578a-48b2-98da-cbbf4a558360 https://github.com/ppy/osu/assets/1329837/dab58718-afa0-4e43-bedb-8d47916c6404
4. The 7K special column is too large, and hard to read. It's fine to be a different colour.

[peppy]

All these are pretty known and should only require minor tweaking or pro-specific changes.

I'll just note that for the hold note bodies being "too long", this is subjective. That said, if we are changing things, the second proposal is the only one I'd be able to get behind, as the first has zero marker of where you are supposed to release.