Upgrade path from TAP13 to TAP14

[jonathanKingston]

As this is a recurring topic elsewhere this needs it's own issue.

This will be a overarching issue for the collective issues (we may break these out into sub issues):

• Discussion of how consumers and producers will handle the upgrade from TAP 13 to 14.
• Backwards compatibility of producers
• Informing the community of the latest changes

[jonathanKingston]

Any support for both formats should be recommendations within the specification itself, backwards compatibility should be advised for at least one prior version.

The specification should be separate to the implementation of the backwards compatibility setup for example there should be no mention of environment variables.

[Krinkle]

@isaacs Hmm was this closure accidental? I see only a typo fix in that commit, possibly related to #31, not the new upgrade section proposed here.

[isaacs]

whoops, that was supposed to close a different PR, my bad.

[isaacs]

31 vs 13 🤦