Change version scheme to <year>.<month>.<patch-level>

Consider changing the versioning scheme of Qt Creator to one that's based on year and month of the (minor) release.

Currently Qt Creator follows semantic versioning (https://semver.org/). This is a well-understood pattern, and you can easily compare different versions.

Anyhow, our current versioning scheme also has issues:

• Qt and Qt Creator version numbers are independent. Anyhow, for the end user this is not necessarily obvious, also because the major-minor value range is similar, often leading to confusion.
• There's no clear rules when to bump the major version number. We haven't since a while, which might lead to the perception that Qt Creator development has stalled. On the other side just bumping it for the sake of it sounds wrong, too.

An alternative proposal is to adapt a <year>.<month>.<patch-level release>, known as Calendar Versioning (https://calver.org/). This keeps most of the advantages of the semantic versioning, but solves the issues outlined above:

• Versions of Qt and Qt Creator are clearly distinguished
• Using year and month makes version numbers are arguably easier to memorize and reason about
• it reinforces our aim for a very steady, regular release schedule

The only issue I see is when a release slips. This could be solved pragmatically though (adapt version number before milestone X, or live with 'incorrect' year.month afterwards), and the exception.