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:

• shields/services/gitea/gitea-helper.js

  Line 30 in 8af909d

  message: `${metric(count)}${messageSuffix ? ' ' : ''}${messageSuffix}`,

• shields/services/dynamic/dynamic-helpers.js

  Line 8 in 8af909d

  suffix: Joi.alternatives().try(Joi.string(), Joi.number()),

• shields/services/downloads.js

  Line 55 in 8af909d

  message: `${metric(downloads)}${messageSuffix}`,

  
  etc

[github-actions]

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

	Messages
📖	✨ Thanks for your contribution to Shields, @chris48s!

Generated by 🚫 dangerJS against 1c833f3

[jNullj]

LGTM