coc: Update Code of Conduct to latest Contributor Covenant

As per the changelog of the Contributor Covenant between versions 1.4
and 2.1, the main changes are:

2.0
- Emphasizes 'community' over 'project' scope, effectively merging the
  Community Covenant into the Contributor Covenant
- Adds expectation about accepting responsibility and apologizing to
  those affected by mistakes, and learning from the experience to the
  list
- Adds sexual attention and advances of any kind as unacceptable
  behavior example
- Moves enforcement responsibilities from project maintainers to
  community leaders
- Adds responsibility to communicate reasons for moderation decisions
  when removing, editing, or rejecting contributions not aligned to the
  Code of Conduct
- Requires community leaders to respect privacy and security of the
  reporter of any incident, not just confidentiality
- Provides a template for code of conduct enforcement

2.1
- Adds 'caste' and 'color' to the preamble

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>