Remove old "New in: rippled X.Y.Z" badges

[mDuo13]

The badges indicating when a feature was added are extraneous and sometimes distracting information when they refer to really old versions. You can't keep synced with the blockchain unless you're running a recent version anyway, so these are only useful for a limited time unless people are updating very outdated code or they're trying to do some sort of historical digging, in which case they should expect to rely on release notes anyway.

As a general policy, we should keep badges that are < ~2 years old (probably about 4-8 scheduled releases), which aligns with the timeline for retiring amendments (XLS-11d).

To avoid too many merge conflicts, this should be done on the IA.v3 branch or on the master branch after merging that one. (It's started in some places already.) We should also update the contributing guidelines to add the above rule/guidance.

[intelliot]

In certain cases (changes of historical importance), perhaps the badge could be replaced with a footnote, which is less distracting than a colored badge.