DEPR: Deprecate cdate_range and merge into bdate_range
- [X] closes #17596
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

A bit going on here, so see summary below.

Deprecated `cdate_range`:
- Removed from api (was added to api after 0.20.3 release)
- Added a `FutureWarning` to `cdate_range` indicating deprecation
    - Added a test for this too
- Converted other tests using `cdate_range` to use `bdate_range` instead
    - Cleaned up the entire test script to make it more consistent with others (i.e. `with` context)
- Removed a reference to `cdate_range` from the current whatsnew (previous commit I wrote)

Expanded functionality of `bdate_range`:
- Added the additional `cdate_range` params (`weekmask`, `holidays`) for custom frequency ranges
- Expanded that functionality beyond what was previously present in `cdate_range`
    - `cdate_range` only supported cday with the `weekmask` and `holidays` params, now all custom freqs are supported.
    - Did this by checking if the passed freq starts with 'C'.
    - Added a test that should catch if a new non-custom freq is added that starts with 'C'.
    - Didn't add this to `cdate_range`; left that as-is.
- Added a `UserWarning` if non-default `weekmask` or `holidays` are passed without a custom freq.
    - Added a test for this too.

Edits to `timeseries.rst`:
- Added a custom freq `bdate_range` example to the "Generating Ranges of Timestamps" section
- Reordered some of the examples in the "Generating Ranges of Timestamps" section
    - Tried to order examples by simplicity
- Otherwise, mostly small rewording changes and other such small fixes (ticks, capitalization, etc.).
