[OTHER] - Redirects out of experimental

[matthewp]

📋 Explain your issue

Redirects RFC was merged and we hope to move it out of experimental. The original PR where it was added is: #3395

Things to do:

• Remove experimental references
• Updated Added in state to reflect when it became stable

[sarah11918]

Thanks, @matthewp !

If nothing in the feature has changed requiring a docs update re: how to use, then I think what we need is:

https://docs.astro.build/en/core-concepts/routing/#configured-redirects-experimental

• remove Experimental
• remove showing the experimental flag in the code sample

In a separate Astro PR to the types file:
https://docs.astro.build/en/reference/configuration-reference/#redirects-experimental

• remove experimental from the header

https://docs.astro.build/en/reference/configuration-reference/#experimentalassets

• remove entirely this experimental flag