chore(main): release 1.6.0-SNAPSHOT (#1961)

## Summary
- Add a marker commit so release-please recognizes the existing
  1.6.0-SNAPSHOT bump
- Without this, release-please thinks it needs to create a snapshot
  PR (to 1.5.1-SNAPSHOT) instead of a release PR for 1.6.0

## Context
The manual `bump snapshot` commit (b1f7ee0) wasn't a conventional
commit, so release-please couldn't parse it and didn't know the
snapshot bump already happened.

## Test plan
- [ ] After merge, verify release-please creates a PR titled
  "chore: release 1.6.0" (not "chore: release main")

Author: zeitlinger