Update statement about release schedule in release.md

[mhucka]

Summarize the task

Issue #6304 mentioned documenting the release process and such things as the release cadence. The process is described in https://github.com/quantumlib/Cirq/blob/main/release.md; however, the section on "Release schedule" currently states

Releases are made on an as-needed basis determined by Cirq maintainers. All Cirq packages (including vendor packages such as cirq-aqt) are released at the same time.

and this should probably be updated to mention the goal of having quarterly releases.

Acceptance criteria - when is the task considered done?

Done when we either update the document or decide we don't want to.

Related

Related issues: #6304.

[pavoljuhas]

csynque meeting - yes, let us add a note about release cadence being approximately every 3 months