textproc/ruby-kramdown-rfc2629: update to 1.5.21

1.5.7 (2021-10-07)

* emergency fix for datatracker bibxml URIs that might trigger caching bug

1.5.8 (2021-10-07)

* Extend /xml -> .xml datatracker workaround to
  kramdown-rfc-cache-i-d-bibxml

1.5.9 (2021-10-14)

* Work around bibxml8 http:// target

1.5.10 (2021-10-15)

* allow parens in DOI

1.5.11 (2021-10-19)

* New diagram tools: "protocol", "protocol-goat"

1.5.12 (2021-10-20)

* aasvg and protocol-aasvg support

1.5.13 (2021-10-29)

* Support auto-linking via abbrevs and # syntax

1.5.14 (2021-10-29)

* provide artwork-{txt,svg}-options for svg tools; Close #135

1.5.15 (2021-10-30)

* Prototype "Discussion Venues" mechanism

  venue:
group: JSON Path
mail: jsonpath@ietf.org
github: ietf-wg-jsonpath/draft-ietf-jsonpath-base

* (add arch: for non-default mail archive and type: for non-default group
  type)

* Also: make noabbrev available as a pseudo-attribute

1.5.16 (2021-11-01)

*  work around gettalong/kramdown#717:
  (line breaks in abbrev usage caused the abbrev to not match)

1.5.17 (2021-11-19)

* Add keys to venue block:
  - `repo` for non-github repos
  - `home` for a WG homepage
* Add json check, json-from-yaml to code block

1.5.18 (2021-11-29)

* Add more keys to venue block:

  - `latest` for a pointer to the latest revision
  - `text` for additional text (sent through markdown;
    can be an array for multiple paragraphs)

* Rename venue note to "About This Document".
* Add datatracker link if a docname is available.
* Increase robustness.

1.5.19 (2021-11-29)

* v3 rfc attributes update
  - Move four "processing instructions" into `<rfc` attributes in v3
  - Handle indexInclude like another one of these PIs
  - Default to consensus: true for cat: std or bcp

1.5.20 (2021-12-01)

* Fix #150 (v2 regression fail): ParameterSet: skip nil in #attrs parameter
  list

1.5.21 (2021-12-01)

* Fix more PI regressions

(taca)