Fix links wrong documentation version

[lauriejim]

Description of what you did:

In migration guide, some links were relative so pointing to the wrong version of the documentation.
I fix this issue.
close #6776

[codecov]

Codecov Report

Merging #6781 into documentation will increase coverage by 0.84%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           documentation    #6781      +/-   ##
=================================================
+ Coverage          19.14%   19.98%   +0.84%     
=================================================
  Files                855      858       +3     
  Lines              11933    12097     +164     
  Branches            1898     1965      +67     
=================================================
+ Hits                2284     2418     +134     
+ Misses              8100     8094       -6     
- Partials            1549     1585      +36     

Flag	Coverage Δ
#front	14.67% <ø> (+0.18%)	⬆️
#unit	41.71% <ø> (+1.77%)	⬆️

Impacted Files	Coverage Δ
packages/strapi-plugin-email/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...n/src/containers/NotificationProvider/constants.js	0.00% <0.00%> (-100.00%)	⬇️
...n/admin/src/containers/LanguageProvider/reducer.js	0.00% <0.00%> (-61.54%)	⬇️
packages/strapi-utils/lib/stringFormatting.js	50.00% <0.00%> (-50.00%)	⬇️
packages/strapi-plugin-graphql/services/utils.js	24.69% <0.00%> (-11.03%)	⬇️
...min/src/containers/NotificationProvider/reducer.js	0.00% <0.00%> (-10.00%)	⬇️
.../admin/src/components/LeftMenuLink/LeftMenuIcon.js	50.00% <0.00%> (-7.15%)	⬇️
...strapi-plugin-upload/config/functions/bootstrap.js	62.06% <0.00%> (-6.69%)	⬇️
...api-admin/admin/src/components/PluginCard/index.js	4.87% <0.00%> (-2.27%)	⬇️
...trapi-admin/admin/src/containers/HomePage/hooks.js	5.00% <0.00%> (-0.89%)	⬇️
... and 160 more

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[alexandrebodin]

LGTM