Rename `postfix` param on `renderVersionBadge()` to `suffix`; test [wordpress]

[chris48s]

This PR renames the postfix param on renderVersionBadge() to suffix. This brings it into line with the terminology we use elsewhere in the codebase. e.g:

• https://github.com/badges/shields/blob/8af909d118d2cc2796b9769baa053fca39ab2f14/services/gitea/gitea-helper.js#L30
• https://github.com/badges/shields/blob/8af909d118d2cc2796b9769baa053fca39ab2f14/services/dynamic/dynamic-helpers.js#L8
• https://github.com/badges/shields/blob/8af909d118d2cc2796b9769baa053fca39ab2f14/services/downloads.js#L55 etc

[github-actions[bot]]

	Warnings
:warning:	This PR modified service code for wordpress but not its test code. That's okay so long as it's refactoring existing code.

	Messages
:book:	:sparkles: Thanks for your contribution to Shields, @chris48s!

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