Implement `draft-ietf-ppm-dap-07`

[tgeoghegan]

DAP-05 is out and Janus should implement it. We need:

• [x] update to new libprio-rs that does VDAF-06 (#1673)
• [x] update to new libprio-rs that does VDAF-07 (https://github.com/divviup/libprio-rs/issues/701; #1811)
• [x] ping pong topology https://github.com/divviup/janus/pull/1811
• [x] deal with error type changes https://github.com/divviup/janus/pull/1853
• [x] verify that we use appropriate media types when serving HPKE configs, aggregate share requests
• [x] HPKE domain separation strings
• [x] update versions table in README.md (https://github.com/divviup/janus/pull/1676)
• [x] Branch for DAP-04 (https://github.com/divviup/janus/pull/1676)
• [x] Daphne is skipping ahead to draft-ietf-dap-09 re-enable Daphne test once they have a DAP-06 implementation
• [x] re-enable divviup-ts tests once it has a DAP-07 implementation (https://github.com/divviup/janus/pull/2388)
• [x] re-enable Prio3Histogram in-cluster test once divviup-api can deal with DAP-05 (https://github.com/divviup/divviup-api/issues/410, #1743)

Draft diff

[tgeoghegan]

DAP-06 is out with a more clear and implementable ping-pong spec, so we'll jump ahead to implementing that.

[tgeoghegan]

The wrong commit got tagged as DAP-06, leaving it with the wrong HPKE domain separation strings and an outdated reference to DAF. So we published draft-ietf-ppm-dap-07 which fixes those issues, and that's what we're going to implement in Janus.