`dpd` can't find out which switch it's connected to via MGS

I deployed 6c3b861 on dublin, and both switch zone shell prompts fail to show which switch we're connected to:

root@oxz_switchnull:~#

There are no services in maintenance and the rack appears to be generally healthy. I can do normal debugging operations, the console is accessible, etc.

The MGS logs show errors due to a client from ::1 failing to provide an api-version header; e.g.,

19:37:58.531Z ERRO a69bd92c-8592-4313-8426-a463c70f45e8 (dropshot): failed to determine request API version
    error = HttpError { status_code: 400, error_code: None, external_message: "missing expected header \\"api-version\\"", internal_message: "missing expected header \\"api-version\\"", headers: None }
    file = /home/build/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/dropshot-0.16.3/src/versioning.rs:67
    local_addr = [::1]:12225
    method = GET
    remote_addr = [::1]:49074
    req_id = 46208126-4acf-480a-a64f-93a86c72db78
    uri = /local/switch-id

I believe what's going on here is:

• #8917 added API versioning to MGS
• progenitor started sending the api-version header in version 0.10.0, which released in April
• omicron pulls in a specific dendrite by commit
• The dendrite commit we're currently pulling in is from Aug 7 (about 3 weeks ago).
• dpd in the dendrite repo uses gateway_client from omicron's main branch.
• On the dendrite commit we're pulling in from Aug 7, the omicron commit it's using for its gateway_client (and other crates) is from February (!!) (https://github.com/oxidecomputer/omicron/tree/e2f9776a2f86de2d4cd35926302bb99cedbdb3d4), which predates progenitor 0.10 and explains why we're not getting an api-version header

Some suggestions from discussions in chat:

• We have a reflector bot that's supposed to open PRs in omicron, but we haven't gotten one for dendrite in quite some time. @augustuswm was going to check on this.
• We don't have automation in dendrite to pull in new omicron bits, but probably should.

Short term plan to fix this:

• https://github.com/oxidecomputer/dendrite/pull/112 updates dendrite's dependency to a new omicron.
• Once that lands, we'll need to update omicron to pull in the new dendrite.

I'm not sure how much stuff is broken by this; maybe not a lot? But seeing switchnull usually means something more serious is broken, which is pretty misleading in this case.

Start with package-manifest.toml and the dendrite dependency revision described in the issue; review dendrite PR #112 and the referenced omicron and Cargo.lock commits. Confirm that the updated dependency sends the api-version header to MGS, then verify that dpd switch-zone shell prompts show the connected switch instead of switchnull.