update changelog dates to 2021 instead of 2020, set date of beta.5 re…

[philvarner]

…lease, update 3 ccs to Proposal instead of Pilot

Related Issue(s): #249

Proposed Changes:

1. update changelog dates to 2021 instead of 2020 (oops!)
2. update changelog date fro beta.5 from TBD to today
3. update new conformance classes (collection, browseable, and children) to be Proposal instead of Pilot as there are no implementations

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.