Downgrade MSRV to 1.80

[zeenix]

Seems 1.81 is too new for some distros but 1.80 is fine. So let's downgrade to 1.80.

[ids1024]

MSRV also seems to be a concern for the atspi crate (used by accesskit): odilia-app/atspi#209 (comment).

(It would definitely help if there was some ecosystem agreement about what MSRV is appropriate to target.)

[luukvanderduim]

Upstream binaries presumably do not care that much, however, with respect to MSRV, the library ecosystem suffers from what Australians call "tall poppy syndrome". It is generally frowned upon to be the first to adopt a later version than some unspoken agreed upon version. As - over time - incrementing MSRV is inevitable anyway, I cheer for those courageous libraries that blink first and increment MSRV because it is unfortunate we hold each other back like this.

We would like atspi to be efficient and have a lighter dependency tree whenever we can, thus we welcomed zbus v4 and now v5. The obvious downside is that some users, notably AccessKit, may not be able to use the latest. It is a difficult problem.