Upgrade Decidim to 0.26

[NicoleeSo13]

This PR upgrades the application to Decidim version 0.26.
Release notes: https://github.com/decidim/decidim/releases/tag/v0.26.0

On deploy it will require to execute the following commands:

• Upgrade server's Ruby version to 2.7.5
• For locked users are present in global search, to update the search and make them disappear, Run in a bin/rails console or create a migration with:
  Decidim::User.find_each(&:try_update_index_for_search_resource)
• For fixing statistic in comments. There's a rake task that you need to run:
  bin/rails decidim_comments:update_participatory_process_in_comments
• For Base64 images migration run the following task:
  bin/rails decidim:active_storage_migrations:migrate_inline_images_to_active_storage[admin_email]

[tramuntanal]

LGTM @NicoleSo13

But you comment in the description that whe must Upgrade server's Ruby version to 2.7.5, but in master we already have Ruby 2.7.5. So this PR is not changing Ruby version. Also I've checked and there is Ruby 2.7.5 in the server.