2.6.X: 'eocw' and 'socw' are invalid

* What command(s) did you run?

```
task XXX modify due:eocw
task XXX modify scheduled:eocw
```

`eocw` is showing up when ZSH command-completing timed-attributes for example either `due:` or `scheduled:` ...

* What did you expect to happen?

Apply the timed modification

* What actually happened?
```
'eocw' is not a valid date in the 'Y-M-D' format.
```

I have checked the `v2.6.1` and `v2.6.0` refspecs for the string `eocw` (and `socw` as well):
```
$ rg -F eocw
ChangeLog
1914: Added as well synonyms soww/eoww plus new socw/eocw for calendar weeks.

scripts/zsh/_task
52: 'eocw:End of calendar week' \

scripts/fish/task.fish
389: 'eocw:End of calendar week' \

doc/man/task.1.in
1102:task ... due:eocw
```

And they don't show up in `.cpp` or `.h` files like `eoww` does (src/libshared/src/Datetime.cpp and bool Datetime::initialize... functions).
But they still appear as valid completions for fish & zsh shells !