Limit the maximum sprint length to 3 months

[quynh1-nguyenthinhu]

The current UI allows the user to set unrealistically long sprint durations, e.g. 10 years.
When the user sets such a long duration, it is mostly by a mistake
But it causes the backlogs plugin to throw errors:

App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Started GET "/redmine/rb/burndown/6/embed" for 172.17.0.1 at 2023-10-04 03:27:47 +0000
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Processing by RbBurndownChartsController#embedded as HTML
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Parameters: {"sprint_id"=>"6"}
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Current user: admin (id=1)
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Rendered plugins/redmine_backlogs/app/views/rb_burndown_charts/show.html.erb (Duration: 740.2ms | Allocations: 448897)
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] Completed 500 Internal Server Error in 766ms (ActiveRecord: 22.2ms | Allocations: 452001)
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75]
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] ActionView::Template::Error (Mysql2::Error: Data too long for column 'burndown' at row 1):
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 13: <script type="text/javascript" language="javascript">
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 14: <%-
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 15: burndown = sprint.burndown
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 16: series = burndown.series.sort{|a, b| l("label_#{a}") <=> l("label_#{b}") }
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 17: dates = sprint.days
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 18:
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] 19: tz = RbIssueHistory.burndown_timezone
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75]
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/models/rb_sprint_burndown.rb:140:in get_burndown' App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/models/rb_sprint_burndown.rb:49:in series'
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/views/rb_burndown_charts/_burndown.html.erb:16
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/views/rb_burndown_charts/show.html.erb:3
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/controllers/rb_burndown_charts_controller.rb:14:in block (2 levels) in embedded' App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] plugins/redmine_backlogs/app/controllers/rb_burndown_charts_controller.rb:13:in embedded'
App 300 output: [3c6d2fb8-f3c0-4e6f-b764-99a1acd54c75] lib/redmine/sudo_mode.rb:61:in `sudo_mode'

And burndown chart cannot be displayed:

Therefore, we limit the maximum sprint length to 3 months to prevent error when display burndown chart of sprint with unrealistically long sprint duration:

[ichylinux]

hi, @quynh1-nguyenthinhu

thank you for this PR.

i have no objection on this!