Update times for community meetings and office hours

[namurphy]

This PR is to update PlasmaPy community meeting times and office hours. This shouldn't be merged yet since the updated meeting times haven't been decided on.

When merged, we'll have to:

• [ ] Update times on README.md on the main branch of PlasmaPy/PlasmaPy
• [ ] Post about this in the Matrix/Element chat

An alternative would be to consolidate the community meeting & office hours, since community meetings are also an opportunity for people to come by.