cloudflare_logpush: Normalise event.severity

[kcreddy]

As noted in #12662, the cloudflare_logpush integration also requires normalising event.severity values according to below scale to conform to Elastic Security detection rules:

21 = Low
47 = Medium
73 = High
99 = Critical

Example implementation for other integrations: #13955

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)