chore: bump io.swagger:swagger-annotations from 1.5.22 to 1.6.16

[dependabot]

Bumps io.swagger:swagger-annotations from 1.5.22 to 1.6.16.

[cursor]

PR Summary

Low Risk
Dependency-only bump of compile-time documentation annotations with no application logic changes.

Overview
Updates swagger_annotations_version in build.gradle from 1.5.22 to 1.6.16, which is the only change in this PR. That version pins the io.swagger:swagger-annotations api dependency consumed by event model classes (e.g. @ApiModelProperty); no source or API surface in this repo is modified.

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