Get IaC production-ready: add branch protections

[bsweger]

Add branch protections to the hubverse-infrastructure repo to prevent inadvertent updates to the main branch and to ensure that updates to hubverse infrastructure go through a review process.

Definition of done:

• [x] Feature branch pushes from org members only
• [x] PRs from org members only
• [x] No pushes directly to main branch
• [x] PRs cannot be merged without at least one reviewer
• [x] PRs cannot be merged until tests and lints have passed

[bsweger]

This was not a coding change, but these settings are now in effect.

• Branch pushes and opening PRs from org members only was already in effect via our organization settings

• Prohibiting pushes to the main branch and requiring reviews and status checks before merging a PR have been enabled via branch protections:

Additionally:

• Turned on the "do not allow bypassing the above settings" to prevent repo admins from overriding the branch protections (this is stricture than our usual branch protections, but PRs to this repo can potentially destroy infrastructure)
• Added some additional repo admins so we don't have a single chokepoint if the branch permissions need to be amended

Lastly, here's what a PR looks like with branch protections enabled: