Remove non-LTS versions from support policy

[shakuzen]

At the time we introduced the concept of non-LTS versions, it made more sense. Time between LTS releases was longer and not a fixed time apart. We also didn't do milestone releases after 1.0 GA. I'm not so sure we need non-LTS anymore or how it would fit with our more regular time-based release cadence now. We are now committing to releasing an LTS version every 6 months, and milestone versions in between there for early testing of the LTS version. What effect a non-LTS version would have on version numbers and when it would even make sense to release a non-LTS version are unclear to me now.