Change XLS exports to XLSX (XLS worksheets limited to 65,536 rows)

Currently the Excel exporter depends on the "spreadsheet" gem: https://github.com/decidim/decidim/blob/63807de254164d47df2073baef5bc9e474063f02/decidim-core/lib/decidim/exporters/excel.rb This gem only exports to XLS-format and will never implement XLSX, as mentioned here: https://github.com/zdavatz/spreadsheet/issues/193 The problem with the legacy XLS format is that it is limited to 65,536 rows per worksheet, see e.g.: https://superuser.com/a/366473 I suggest to change the "spreadsheet" dependency to something that writes XLSX instead, e.g. the rubyXL gem suggested in the linked GitHub issue above. The change should be rather simple because of the simplicity of the exporter code. I believe the "spreadsheet" gem dependency is not used anywhere else than in the linked exporter.