Pin version of minify to 2.5.1

Pin minify to 2.5.1 to resolve issue arising in 2.5.2.

Minify was introduced in !633 (merged).

This MR fixes the error with the new minify version https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/361043305.

201 github.com/dustin/go-humanize
202 golang.org/x/sys/unix
203 github.com/fsnotify/fsnotify
204 github.com/matryer/try
205 github.com/spf13/pflag
206 github.com/tdewolff/parse/buffer
207 github.com/tdewolff/parse
208 github.com/tdewolff/parse/strconv
209 github.com/tdewolff/minify
210 github.com/tdewolff/parse/css
211 github.com/tdewolff/minify/css
212 github.com/tdewolff/parse/html
213 github.com/tdewolff/minify/html
214 # github.com/tdewolff/minify/html
215 src/github.com/tdewolff/minify/html/html.go:350:12: undefined: parse.ReplaceEntities
216 src/github.com/tdewolff/minify/html/html.go:350:39: undefined: html.Entities
217 github.com/tdewolff/parse/js
218 github.com/tdewolff/minify/js
219 github.com/tdewolff/parse/json
220 github.com/tdewolff/minify/json
221 github.com/tdewolff/parse/svg
222 github.com/tdewolff/parse/xml
223 github.com/tdewolff/minify/svg
224 github.com/tdewolff/minify/xml
225 # github.com/tdewolff/minify/xml
226 src/github.com/tdewolff/minify/xml/xml.go:149:11: undefined: parse.ReplaceEntities
227 src/github.com/tdewolff/minify/xml/xml.go:149:38: undefined: xml.Entities