[clang] [clang] ``README.txt``: Replace the link to the old bug tracker with the new one. (PR #93878)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM! Do you need someone to land this on your behalf?

https://github.com/llvm/llvm-project/pull/93878