Rename `gen_ai.system.message` to `gen_ai.instructions` or similar

[lmolkova]

'System' is used in model APIs and 'instruction' is a wider term that's more popular in agents. Given that instructions is more precise, we should consider renaming corresponding event.

[aabmass]

@lmolkova is this one still needed? I think we discussed for #2179 and decided to go with gen_ai.system_instructions

[lmolkova]

good catch! closing - implemented in #2179