Remove `beacon` field in certificate

[jpraynaud]

Why

Remove the beacon field from the certificate which is optional for the client since distribution 2430.

What

Remove the beacon field from the certificate by following the era_deprecate!("Remove immutable_file_number"); in the code

How

• Remove the Beacon field from the certificate
• Assess if we need to recompute the certificate chain
• Adapt explorer
• Prepare announcement breaking change in the next distribution