Log Message:
tex-pgf{,-doc}: update to 3.1.10

- `\pgfversiondatetime` and `\pgfrevisiondatetime` have been removed.
  `\pgfversiondate` and `\pgfrevisiondate` now use the format `YYYY-MM-DD`.
  `\pgfrevision{,date}` and `\pgfversion{,date}` are synonyms for now, but the
  revision should eventually gain back its original meaning.  However, as of now
  this is not supported by l3build.
- Many operations in `pgfkeys` used to use `\csname` directly which lets the
  given csname become `\relax` in case it wasn't defined. This resulted in some
  leakage of accidentally `\relax`ed keys into the global scope. This has been
  cleaned up a little. To preserve compatibility macros that used to expand to a
  `\relax`ed csname now expand to a primitive `\relax`. This affects the
  user-level commands `\pgfkeysgetvalue` and `\pgfkeysgetvalueof`. For the
  former the change should not be visible except for the number of expansions
  required. For `\pgfkeysgetvalueof`, however, the behavior is manifestly
  different in that it will now expand to an alias for the primitive `\relax` in
  case the value is undefined instead of a `\relax`ed csname. It has always been
  semantically wrong to assign to the result of `\pgfkeysgetvalueof`, but now it
  will have undesired side-effects. Therefore this change is breaking.
- The `textures` and `vtex` drivers have been deprecated. Both engines are no
  longer in active development and lack eTeX features which are required for
  quite some time in PGF.
- The file `pgfutil-common-lists.tex` is deprecated and therefore no longer
  `\input` by `pgfutil-common.tex`. The macros from this file are specifically
  meant for pgfplots and are not used in PGF at all.

### Added
- l3build support for packaging PGF/TikZ
- Add Matrix chat to README
- Add rhombic polihedra #1022
- Add Developer Certificate of Origin (DCO) to Pull Request template and enforce
- Add test set for `graphdrawing` (gd)
- pgfkeys gained support for loading libraries
- Add dependabot to keep GitHub Actions up to date