Update TSC General meeting announcement workflow to reflect new schedule

[cdils]

This call is now moving to a monthly call. The current time remains the same, but the trigger needs to change to the second Thursday of the month.

Workflow: https://github.com/fairpm/.github/blob/ca8faf32b8c400a577d011a7004999ab76515d70/.github/workflows/create-tsc-call-discussion.yml