Make requirement_level required

[trisch-me]

Explicit is better than implicit.

Recently while moving attributes to the registry we have started to add also requirement_level everywhere to make it explicit.

I propose to make it even required as we recently did with stability and enforce through build-tools.

[joaopgrassi]

I support this! I also prefer it to be explicitly set than relying on defaults/omitted.

[lmolkova]

Should we expand the decision to everything else going forward? I.e. no default values (except null for note and other fully optional things)?