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Use Golang 1.18 (as minimal requirement) for Gitea 1.17 release, make sure the Golang version is still actively supported during Gitea 1.17 lifecycle.
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>