CI: Add Ruby 3.4, Ruby 4.0

[olleolleolle]

This PR adds current Ruby versions to build matrix.

[olleolleolle]

Tests have a flake that looks like

  1) Error:
SimpleXlsxReader::parse errors#test_0002_records a load error if not configuration.catch_cell_load_errors:
Errno::ENOENT: No such file or directory @ rb_sysopen - /tmp/workbook20260318-2354-7cwqdt.xlsx
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/input_stream.rb:128:in `initialize'
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/input_stream.rb:128:in `open'
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/input_stream.rb:128:in `get_io'
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/input_stream.rb:56:in `initialize'
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/entry.rb:659:in `new'
    vendor/bundle/ruby/3.1.0/gems/rubyzip-3.2.2/lib/zip/entry.rb:659:in `get_input_stream'
    lib/simple_xlsx_reader/loader.rb:90:in `add_sheet_parser_at_index'
    lib/simple_xlsx_reader/loader.rb:62:in `read'
    lib/simple_xlsx_reader/loader.rb:11:in `init_sheets'
    lib/simple_xlsx_reader/document.rb:20:in `sheets'
    <internal:kernel>:90:in `tap'
    lib/simple_xlsx_reader.rb:45:in `open'
    test/simple_xlsx_reader_test.rb:803:in `block (3 levels) in <top (required)>'