changeable_budget compatibility

[lmoureaux]

Is your feature request related to a problem? Please describe.

In #1404, the changable_tax ruleset parameter (in cities.ruleset) to changable_budget. This changed the ruleset format in an incompatible way. Moreover, the spelling is incorrect and should be changeable.

Describe the solution you'd like

• Accept changable_tax, but emit a warning that it's deprecated.
• Rename changable_budget to changeable_budget. While this is another format change, I think it's an acceptable tradeoff because changable_budget was only present for a short time.

The new syntax should be preferred over the old one.

Describe alternatives you've considered

Also introducing a fallback for changable_budget. I don't think it's necessary.

Additional context

@jwrober tried to load Aviation in pyciv, which cares about this kind of errors (unlike the main game).