[Nomination] Add ST representative to Security group (#93176)

I'd like to nominate myself to join the LLVM Security group as a
representative of ST. I work in ST's compiler team contributing to
upstream (LLVM and GNU) and several downstream toolchains. We believe
that it is important for us to be part of this group to address or
report any potential security issues the LLVM project or our toolchains
may encounter.