Replace form_enctype(form) with form_start(form).

Since form_enctype(form) is deprecated since 2.3 form_start(form) should be used.
bug #4603 Replace form_enctype(form) with form_start(form). (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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Replace form_enctype(form) with form_start(form).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >=2.3
| Fixed tickets |

Since form_enctype(form) is deprecated since 2.3 form_start(form) should be used.

Commits
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21d69eb Replace form_enctype(form) with form_start(form).