Use Unicode copyright symbol (in UTF-8 encoding) in all source files

It is now supported by the MATLAB editor (as of R2022a).

The old ASCII notation is left in some files that we copy as-is from other
sources (e.g. in the contrib/ and m4/ subdirectories).

The particles submodule is not updated at this point, because it is in an
inconsistent state.

[skip ci]