Update CI workflow link in README

[GuroChil]

Changes made:
File: axum-core/README.md

• Old: https://github.com/tokio-rs/axum-core/actions/workflows/CI.yml

• New: https://github.com/tokio-rs/axum/blob/main/.github/workflows/CI.yml

Reason for changes:
Updating the CI workflow link to point to the correct location in the main repository's .github/workflows directory. The old link was pointing to a non-existent path.

If there's a more appropriate or preferred link for the CI workflow, please let me know and I'll update the PR accordingly.

[jplatte]

I don't really know why we have CI badges to begin with. GitHub shows the CI status on each commit in the history.

I'd prefering removing this and any others we have. Thoughts @yanns @mladedav @Turbo87 ?

[mladedav]

When they're available, I sometimes use e.g. the crates.io one as a hyperlink to get to the crate's page. So I'd be personally in favor of getting rid of the CI (unless someone has a usage for it) but I'd rather keep the others, though I don't feel strongly about it.

@GuroChil how did you find the broken link, did you try to use it for some purpose or were you just hunting specifically for broken links to fix (I noticed you opened similar PRs in tracing at the same time)

[Turbo87]

fully agree, IMHO they're just unnecessary noise at this point

[jplatte]

@mladedav I was specifically taking about CI badges, I've also found other badges useful (though haven't clicked on in a long time).

[mladedav]

Ah, sorry, I must have misinterpreted the "remove this and any others we have", I thought you were talking about all of the badges. In that case I agree.

[jplatte]

Ah yeah, I was just taking about CI badges in any of the other readme files.

[yanns]

I don't use this link. It can be removed IMO.

[jplatte]

@GuroChil want to update this PR to delete the CI badges from axum, axum-core, axum-extra and axum-macros' README.mds?