Modernize to use keras3

[jemus42]

Tried to fix a bunch of stuff, from NEWS.md:

• Switched from deprecated keras package to keras3.
• Use py_require("tensorflow") on package load to ensure TensorFlow is available.
• Updated loss function to use keras3 ops (op_sum, op_log, op_clip) instead of legacy backend() API.
• Updated optimizer_rmsprop() to use learning_rate argument (replacing deprecated lr).
• Updated regularizer_l2() to use l2 argument (replacing deprecated l).
• Removed CUDNN_LSTM and CUDNN_GRU RNN types (regular LSTM/GRU auto-use CuDNN in modern TensorFlow).
• Wrapped examples in \donttest{} and added test skips for environments without TensorFlow.
• Fixed DESCRIPTION metadata (Authors@R, title case, encoding, complete description).
• Used \doi{} instead of \url{} for DOI references in documentation.

Long term we should maybe just point people to survdnn and archive this repo, but in the meantime the unusable state of the package was bugging me, see R universe checks.