Number element with step and min is not supported

Problem/Motivation

On a form with a number element with a #step and a #min, the offset induced by #min is not taken into account.

Steps to reproduce

Example:
min = 5
step = 10

Authorized values are: 5, 15, 25, 35, etc.

For clientside validation, authorized values are: 10, 20, 30, 40, etc.

entering 10 allows to pass clientside validation but serverside (core) validation is not OK.

Proposed resolution

Remaining tasks