You can use the timezone option in the date filter.
Remove your ruby filter as it is just override the logts field with the current utc time and try this filter.
date {
match => ["logts", "ISO8601"]
timezone => "your/timezone"
}
IST is ambiguous, it could mean India Standard Time, Irish Standard Time or Israel Standar Time, you need the name of the timezone you are, which would be Asia/Kolkata, Europe/Dublin or Asia/Jerusalem for each one of those cases.
In the case IST means India Standard Time you would use:
date {
match => ["logts", "ISO8601"]
timezone => "Asia/Kolkata"
}