[boost] PSA: GitHub public RSA key has changed

Hi, This is just a heads up that GitHub public RSA key has changed. If you're using ssh with an RSA key to access GitHub, you may see errors saying that remote host identification has changed. The change was made on March 24. You can find more details and instructions to update your locally cached key in the link below. https://github.blog/2023-03-23-we-updated-our-rsa-ssh-host-key/ PS: This may affect CI runs if they are configured with the ssh-key option, but I don't think any of the Boost libraries are.