Added troubleshooting info about Last name can't be blank error

What does this MR do and why?

We have several old open issues about encountering Last name can't be blank error in Compute Minutes purchase flow on gitlab.com:

• Unable to buy Pipeline minutes - “last name can... (#431926 - closed)
• Last name can't be blank - when trying to buy ... (#414483 - closed)
• https://gitlab.com/gitlab-org/gitlab/-/issues/422802+

This error is an edge case, and only happens for users that have one name in the Full name field. This is an edge case, as it is not possible to created a profile without the first name and last name anymore.

We are not going to resolve this issue in the gitlab.com purchase flow, because we are moving it to the Customers Portal (where this issue does not exist) within the next few months.

This MR adds troubleshooting instructions to these Docs in the meantime.

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changed milestone to %17.2

added groupsubscription management label

assigned to @tgolubeva

added Category:Subscription Management devopsfulfillment sectionfulfillment labels

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	1 Message
	This merge request adds or changes documentation files. A review from the Technical Writing team before you merge is recommended. Reviews can happen after you merge.

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mentioned in issue #431926 (closed)

Proper labels assigned to this merge request. Please ignore me.

@tgolubeva - please see the following guidance and update this merge request.

	1 Error
	Please add typebug typefeature, or typemaintenance label to this merge request.

added docs-only label

mentioned in epic &5785 (closed)

added 1 commit

• 15db09cd - Fixed lint errors

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