[Ruby SDK] Upgrades OpenAPI Generator from 7.12.0 to 7.26.0

[gbiernacki-dropbox]

Summary

Upgrades OpenAPI Generator from 7.12.0 to 7.26.0 for the Ruby SDK. No breaking changes to the public API.

Upstream template changes

Change	Upstream PR	Version
Models inherit from a shared ApiModelBase class (to_s, to_body and ._deserialize are inherited instead of generated in every model)	#22052	7.17.0
New class methods acceptable_openapi_types and acceptable_openapi_nullable, including attributes inherited from allOf parents	#24892	7.26.0
Simplified path parameter replacement	#23485	7.22.0
Validation message typo fixed ("great than" → "greater than")	#21287	7.14.0
anyOf response types are built with build	#22392	7.18.0
RuboCop TargetRubyVersion set to 2.7, matching the gemspec	#24992	7.26.0

Existing custom behaviour is kept: to_hash(include_nil = true), instance-level build_from_hash and _deserialize.

Release

This will ship as a minor release (1.16.0):

• No breaking changes: existing public methods and signatures are unchanged, dependencies are unchanged, and the minimum Ruby version stays >= 2.7.
• New public API is added (ApiModelBase and the acceptable_openapi_* class methods), which is backwards-compatible.