lwtnn/lwtnn: Version 2.12

This release fixes a number of bugs:

• Properly handle InputLayer in Keras Sequential models (thanks @QuantumDancer)
• Fix bug supporting TimeDistributed BatchNormalization layers
• Fix bugs in lwtnn-split-keras-network.py
• Fix some annoying warnings in compilation
• Fixes for compilation errors in gcc11 (thanks @matthewfeickert)
• Replace broken link for boost in the minimal install (now getting boost from a public CERN URL)

There were some tweaks to reduce element-wise calls through std::function in activation functions. This should mean a lot less pointer dereferencing.

There were also some improvements to overall code health, all from @matthewfeickert:

• Add Github Actions based CI
• Code linting, also add pre-commit hooks to run linting, and enforce some of this in CI
• Expand text matrix to include C++17, C++20, and gcc11
• Specify dependency versions more carefully, increase the range of allowed CMake versions