fix to_s method error usage (Redmine 6 compatibility)

[mayasd]

Since we upgraded to Redmine 6 we got an error when displaying recurring issue (root issue only)

[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] ActionView::Template::Error (wrong number of arguments (given 1, expected 0)):
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] 
Causes:
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] ArgumentError (wrong number of arguments (given 1, expected 0))
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     15: 
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     16:   <% RedmineRecurringTasks::IssuePresenter.new(@issue).tap do |issue| %>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     17:     <p><strong><%= l(:schedule) %></strong></p>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     18:     <%= issue.schedule %>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     19:     <%= issue.schedule_template %>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     20:   <% end %>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]     21: </div>
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c]   
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/lib/redmine_recurring_tasks/issue_presenter.rb:16:in `to_s'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/lib/redmine_recurring_tasks/issue_presenter.rb:16:in `block in schedule'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/lib/redmine_recurring_tasks/issue_presenter.rb:13:in `map'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/lib/redmine_recurring_tasks/issue_presenter.rb:13:in `schedule'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/app/views/recurring_tasks/issues/_bottom.html.erb:18
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] plugins/redmine_recurring_tasks/app/views/recurring_tasks/issues/_bottom.html.erb:16
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook/view_listener.rb:62:in `block (2 levels) in render_on'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook/view_listener.rb:60:in `map'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook/view_listener.rb:60:in `block in render_on'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook.rb:66:in `block (2 levels) in call_hook'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook.rb:66:in `each'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook.rb:66:in `block in call_hook'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook.rb:63:in `call_hook'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/hook.rb:101:in `call_hook'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] app/views/issues/show.html.erb:108
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] app/controllers/issues_controller.rb:120:in `block (2 levels) in show'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] app/controllers/issues_controller.rb:112:in `show'
[274a3e25-8c32-4d15-8d52-d9dc07e9ee5c] lib/redmine/sudo_mode.rb:78:in `sudo_mode'

Problem occur when to_s method is call with an argument in line 10 and 16 (depending on recurring type) from lib/redmine_recurring_tasks/issue_presenter.rb file.

Because date is always the same (2000-01-01), I replace to_s method with strftime method to display only hours/minutes/TZ informations.