Update serverless technical limitations

[jmikell821]

Topic: https://docs.elastic.co/serverless/security/security-technical-preview-limitations
Change from:

Data ingest rate, total of all data sources, greater than 100GB per day

to

Data ingest rate, total of all data sources, greater than 500GB per day

[jmikell821]

Merged https://github.com/elastic/staging-serverless-security-docs/pull/347.