Update OSGeo hex and guidelines

[adamjstewart]

Very slight change in color, also added links to document the source.

Before

After

[PeterShaggyNoble]

Thanks, @adamjstewart 👍🏻 Did you do a comparison between our current icon and the new source to make sure they line up perfectly? If so then just the one small change below and this is ready to merge.

[adamjstewart]

I did not compare the new and old source to make sure they're the same, I only updated the URL. I'm happy to undo that change if you want me to.

[PeterShaggyNoble]

I noticed a lot of other guidelines use commit hashes for stability/reproducibility. Is this a new policy? I'm fine with whatever you decide.

Yeah, after looking at a few more entries, it seems we've become super-inconsistent about whether we use the default branch or the commit hash for the guidelines. We'll need to sort that out and address it. For this one, though, I've since found this page on their website, which I think would work better for the guidelines as it's more likely to be kept up to date with any future changes.

I did not compare the new and old source to make sure they're the same, I only updated the URL. I'm happy to undo that change if you want me to.

If we're not at least doing a comparison against the new source then, yes, we should keep our existing one.

[PeterShaggyNoble]

Thanks, @adamjstewart 👍