Improve validation of providers

[ml-evs]

e.g.

optimade-python-tools/optimade/models/optimade_json.py

Lines 203 to 206 in 506373c

	prefix: str = StrictField(
	...,
	description="database-provider-specific prefix as found in section Database-Provider-Specific Namespace Prefixes.",
	)

should only match [a-z][0-9] , as pointed out by @CasperWA elsewhere.