docs: update CHANGELOG with releases v15.0.0 through v25.0.0

[emmaeng700]

Fixes #4465

CHANGELOG.md was last updated at v14.0.0. Added entries for all
releases from v15.0.0 to v25.0.0 including:

• Breaking changes introduced in v20.0.0 (Java 8 removal, optional
  param consolidation, legacy suffix module)
• Full feature and bugfix details for v25.0.0
• API regeneration notes for intermediate releases

[linux-foundation-easycla]

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• ✅ login: emmaeng700 / name: Emmanuel Oppong (59a605c)

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[yue9944882]

/lgtm
/cc @brendandburns

[brendandburns]

/lgtm
/approve

I do worry about having the release notes in two places (Github releases and CHANGELOG) we should see how we can keep them in sync going forward.

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This pull-request has been approved by: brendandburns, emmaeng700

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