[FSSDK-12876] Prepare for release go-sdk v2.5.0

[Mat001]

Summary

Prepare for release v2.5.0 and fix the version/changelog gap left by v2.4.0.

Release Details

• Version Bump: 2.3.0 → 2.5.0 in pkg/event/version.go.
  • Note: the version was never bumped for v2.4.0, so master (and the v2.4.0 tag) still reported 2.3.0. This corrects it directly to 2.5.0.
• CHANGELOG.MD:
  • Backfilled the 2.4.0 entry (v2.4.0 shipped without updating the changelog file; content matches the existing GitHub v2.4.0 release notes).
  • Added the 2.5.0 entry.

2.5.0 changes (v2.4.0...master)

• FSSDK-12369 Add local holdouts support to Go SDK ([AI-FSSDK] [FSSDK-12369] Add local holdouts support to Go SDK #451)
• FSSDK-12670 Block ODP identify events with single identifier ([AI-FSSDK] [FSSDK-12670] Block ODP identify events with single identifier #452)
• FSSDK-12760 Add localHoldouts to datafile for backward compatibility ([AI-FSSDK] [FSSDK-12760] add localHoldouts to datafile for backward compatibility #453)
• FSSDK-12813 Normalize decision event campaign_id, variation_id, and entity_id ([AI-FSSDK] [FSSDK-12813] Normalize decision event campaign_id, variation_id, and entity_id #455)
• FSSDK-12750 Use attribute ID instead of key for CMAB prediction requests ([AI-FSSDK] [FSSDK-12750] Use attribute ID instead of key for CMAB prediction requests #456)

Follow-up

• Supersedes/closes stale PR [AI-FSSDK] prepare for release go-sdk v2.4.0 #449 (prepare v2.4.0), which was never merged even though v2.4.0 was released.

FSSDK-12876

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[jaeopt]

LGTM