Update 'Twitter' to 'X' in footer of homepage

[BishalJena]

From this:

to this:

[babel-bot]

Hey @BishalJena! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
community that typically always has someone willing to help. You can sign-up here
for an invite.

[BishalJena]

I can work on this issue quickly and raise a PR.

[nicolo-ribaudo]

Even though the official name is "X", I find "Twitter" (which everybody recognizes) in that list to be much more descriptive. Given that it's not a list of social networks (e.g. Facebook, Linkedin, X, Instagram), X could mean literally anything.

[BishalJena]

Let's keep the name as "Twitter" and just update the backlink from "twitter.com/babeljs" to "x.com/babeljs".
How about that?

[seaisbig]

make it as ''twitter/x''