Use github.com for redirect service

[lseppala]

github-redirect.dependabot.com has a single CNAME record pointing to github.com.

❯ dig +all github-redirect.dependabot.com
; <<>> DiG 9.10.6 <<>> +all github-redirect.dependabot.com

;; QUESTION SECTION:
;github-redirect.dependabot.com.	IN	A

;; ANSWER SECTION:
github-redirect.dependabot.com.	3600 IN	CNAME	github.com.
github.com.	60	IN	A	140.82.113.18

This change updates the DEFAULT_GITHUB_REDIRECTION_SERVICE constant from github-redirect.dependabot.com to github.com. This DNS entry is a stronger candidate for long-term support as dependabot.com records are decommissioned.

It wasn't strictly necessary to update tests as github_redirection_service is explicitly set in tests, but references to github-redirect.dependabot.com were replaced to avoid future confusion.

[brrygrdn]

This change makes sense to me, it is worth testing that GitHub doesn't treat github.com URL as mentions, which we can do here:

#4441
https://github-redirect.dependabot.com/dependabot/dependabot-core/pull/4441
https://github.com/dependabot/dependabot-core/pull/4441

This looks good to me based on the preview, let's make sure it works when I submit...

@brrygrdn's comment should take precedence over my approval

[brrygrdn]

🚀

I was 99% this did the right thing, but I just wanted to check 😅 This is a good call as it's one less dependency on the dependabot.com DNS.