GitHub: Can no longer search code without being logged in.

Response from Martin Woodward, GitHub's VP of Developer Relations:

Sorry for the inconvenience @koepnick - while searching across all repos has required being logged in for a long time, when we enhanced the search capabilities earlier in the 2023 we had to extend this to repos as well (see https://github.blog/changelog/2023-06-07-code-search-now-requires-login/).

This is primarily to ensure we can support the load for developers on GitHub and help protect the servers from being overwhelmed by anonymous requests from bots etc.