New icon for gather farming upgrade

[Langbart]

Until [26675] the olive tree was used for gathering food, since then it is only used for wood.

The small symbol for gaterhing upgrade in the farmhouse represents an olive, it doesn't fit anymore (maybe it never really did). See image below.

Related IRC 0ad-dev conversation ​19/Mar/22

16:53 < Langbart> marder u made a patch for olive tree?
16:53 < marder> yup
16:53 < Langbart> should the icon change?
16:55 < marder> hmmmmm. yes? if there are any better candidates... But tbh, it didn't fit before either :D olive =/ grain
16:55 < marder> so could also stay until something better is available

Here are the current icons for improving food collection.

possible solution

I like the tech upgrade numbering, where a little I,II, III was added to the corner.

So an idea would be to use the first or second icon and just a number in the corner.