Fix merge_method getter and setting methods

What does this MR do and why?

Failed pipeline: https://jihulab.com/gitlab-cn/gitlab/-/pipelines/1339292/failures

Bug from: !1711 (merged)

• Add squash option checking to merge_method method;
• Remove useless project validate method (to fallbacks to fast-forward);
• Fix merge_method= method.

Screenshots or screen recordings

NonUI

How to set up and validate locally

MR acceptance checklist

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