Log Message:
tex-l3kernel{,-doc}: update to 2021

### Fixed
- Loading when `\expanded` is not available

### Added
- `\msg_note:nnnnnn`
- `\str_compare:nNnTF`
- `\sys_timer:`
- `\prop_concat:NNN`, `\prop_put_from_keyval:Nn`
- Functions to show and log various datatypes
  `\coffin_show:Nnn`, `\coffin_show:N`, `\coffin_log:Nnn`, `\coffin_log:N`,
  `\color_log:n`, `\group_show_list:`, `\group_log_list:`,
  `\ior_show:N`, `\ior_log:N`, `\iow_show:N`, `\iow_log:N`,
  `\tl_log_analysis:N`, `\tl_log_analysis:n`
- `\legacy_if_set_true:n`, `\legacy_if_set_false:n`, `\legacy_if_set:nn`

### Fixed
- Checking brace balance in all regex functions
- Removing duplicates in clists when items contain commas

### Changed
- Slight speed up in some elementary int/dim/skip/muskip operations and
  in setting tl or clist variables equal.
- Speed up mapping functions in l3clist, l3prop, l3seq, l3tl

### Added
- `\cctab_item:Nn`
- `\clist_use:nnnn` and `\clist_use:nn`

### Fixed
- Loading of backend in generic DVI mode
- Make `\keyval_parse:nnn` alignment-safe
- Control sequences and category codes in regex replacements

### Changed
- Speed up \group_align_safe_begin:

### Added
- Color export in comma-separated format
- `\ur{...}` escape in `l3regex` to compose regexes
- `\seq_set_split_keep_spaces:Nnn`
- `\seq_set_item:Nnn(TF)` and `\seq_pop_item:NnN(TF)`
- `\box_ht_plus_dp:N`
- `\clist_map_tokens:nn`, `\clist_map_tokens:Nn`,
  `\str_map_tokens:nn`, `\str_map_tokens:Nn`

### Changed
- Use prevailing catcodes instead of string in regex replacement
  (*Breaking change*)
- `\__kernel_file_name_sanitize:n` now uses a faster `\csname`-based
  approach to expand the file name
- Improved performance for basic conditionals
- `\pdf_version_gset:n` support for `dvips`
- Improve handling of `\exp_not:n` in `\text_expand:n`
- `\file_full_name:n` now avoids calling `\pdffilesize` primitive multiple times
  on the same file
- Show printable characters explicitly in `\regex_show:n`
- Regex replacement now errors when using a submatch (`\1` etc) for which
  the regex has too few groups
- Showing complex datatypes now validates their internal structure
- Indexing in l3doc: all page references before codeline references,
  improve target placement, solve pdfTeX and makeindex warnings

### Fixed
- Evalutate integer constants only once
- Detect `\ior_map_inline:Nn` calls on undefined streams

### Deprecated
- `l3docstrip` converted to a stub which simply loads DocStrip: use
   the latter directly