DOCSP-30987: use FCV instead of fCV

[jwilliams-mongo]

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Pull Request Description

replacing fCV with FCV

JIRA URL: https://jira.mongodb.org/browse/DOCSP-30987

Staging URL:

• https://deploy-preview-178--mongodb-docs-meta.netlify.app/style-guide/terminology/alphabetical-terms/#std-term-FCV
• https://deploy-preview-178--mongodb-docs-meta.netlify.app/style-guide/style/capitalization/#capitalize-most-abbreviations

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[corryroot]

LGTM with a suggestion for the preexisting text. Thanks @jwilliams-mongo!

[stephmarie17]

LGTM with the suggestion from @corryroot ✅