Fix missing ModelAdmin menu_icon for Wagtail 2.11+

[benoitvogel]

I noticed that the "form" icon in the main menu is missing from Wagtail 2.11 onwards. I'm not sure which commit between 2.10.2 and 2.11 is at fault but here's my PR to fix it.

Looks like simply changing the value to "form" also works before 2.11 but I wanted to make sure that I don't break anything so I relied on Wagtail's VERSION.

Not sure if this trivial change needs some testing (TBH i also don't know how to test Wagtail icons).

Please let me know if somethings is wrong or missing.

[codecov[bot]]

Codecov Report

Merging #171 (ba799e2) into master (37f7cb1) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   95.65%   95.65%           
=======================================
  Files          28       28           
  Lines         943      944    +1     
  Branches       85       85           
=======================================
+ Hits          902      903    +1     
  Misses         34       34           
  Partials        7        7           

Impacted Files	Coverage Δ
wagtailstreamforms/wagtail_hooks.py	87.70% <100.00%> (+0.10%)	:arrow_up:

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[leongraumans]

Hi @benoitvogel,

Thanks! I will close this PR as the missing menu icon is fixed by https://github.com/labd/wagtailstreamforms/commit/8861656a2b0bce2c1913e74f15af2f6c4fd765a7. I skipped the version-check, since that's not relevant anymore.

Will create a release later this week.