DOC Add entry about utf8mb4 encoding update to changelog

[maxime-rainville]

Description

Add entry about utf8mb4 encoding update to changelog

Issues

• CMS6: Default to utf8mb4 silverstripe-framework#11336

Related PRs

• ENH Default DB settings to use 4 bytes to store UTF8 characters silverstripe-framework#11431
• ENH Remove old logic to default UTF8 DB setting to 4 bytes recipe-core#102

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[maxime-rainville]

The linting is failing ... but because of the 5.3rc changelog.

[GuySartorelli]

Lgtm