Update logos for Actix and Rust

[lcian]

This provides a white background as the 80x80 logo for Actix instead of the black background we already have in /svg
It makes it consistent with the 80x80 icon for Rust
Source: actix/actix-website#18

[cleptric]

this won't work on dark mode

[lcian]

this won't work on dark mode

True, thanks for pointing that out @cleptric
I've updated the PR to use a white background as we do for https://github.com/getsentry/platformicons/blob/master/svg_80x80/rust.svg
to at least make it consistent with it

[lcian]

As discussed, we're going to update both Actix and Rust so that they have a consistent white background, and display nicely on dark mode as well.