THRIFT-5903: remove the Fixnum as it is fully removed from 3.2

[xuan-cao-swi]

Remove the Fixnum since it's no longer a valid constant from ruby 3.2.

• Did you create an Apache Jira ticket? THRIFT-5903
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[kpumuk]

See also: #3249

[xuan-cao-swi]

Close as this is a duplicate