Add a PLEP that describes the support policy for releases and LTS releases

[namurphy]

It would be helpful to have a PLEP that answers the following questions:

• How long would feature releases be supported (with bugfixes, etc.)?
• How often will long-term support (LTS) releases be made?
• How long will LTS releases be supported?

This would likely be similar to Astropy Enhancement Proposal 2. Having an LTS policy could end up being really helpful for facilities who depend on PlasmaPy in order to have an extra level of stability (though we should also encourage packages that depend on PlasmaPy to test against the main branch of PlasmaPy and/or our monthly beta releases).

Thinking of common practices from the pythoniverse, it's common for feature releases to be supported either for a specific period of time (like 6 or 9 months) or until the next feature release. LTS releases are often made every two years, and supported for somewhere between ∼2–3 years.

If we follow the schedule in [NEP 29], we'll be dropping support for a Python version around April of each year. One possibility would be to have the last release that works on Python 3.9, 3.11, ... be an LTS release... like 2024.03.*, 2026.03.*... if we time LTS releases for every other February. That way, if someone is using a slightly older version of Python, they'll have access to bugfixes.

We might also want to time scheduled removal of features to the first release following an LTS release.

We'd probably want to mention how LTS releases treat features that are explicitly labeled as prototypes.