SITL: In SIM::Battery, add "_amps" suffix on current variable names

[hunt0r]

Summary

Simply rename battery current variables current to current_amps.
It is trivially "PR-chained" behind #32808 . (I will rebase for clarity when that one lands.)

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

This is confirmed correct by successfully compiling and also SIM::Battery's unit tests.