Replace freq '1M' with '1MS'

[AdamRJensen]

• I am familiar with the contributing guidelines
• Tests added
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
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The freq='1M' is used in a couple of places in the pvlib iotools functions, which results in the following future warning:

FutureWarning: 'M' is deprecated and will be removed in a future version, please use 'ME' instead.
months = pd.date_range(

The frequency 'M' is being phased out in favor of 'ME' and 'MS' which correspond to month end and month start. 'M' is equivalent to 'ME'. However, 'MS' is the only one supported in our minimum Pandas version, whereas 'ME' was first introduced in a later version. For the iotools functions, I think 'MS' is conceptually more correct.

Minimum changes to the lookup linked turbidity functions were made to make tests pass. I preferred making these modifications over calculating new test values.

[AdamRJensen]

I ended up removing the label parameter in the cams iotools function. Note, no other iotools function supports this and it turned out to be a maintenance burden that I don't want to support in the future. This means that this PR should be made in a .0 release (for this reason I'm holding off adding a whatsnew entry).

Note, test failures are unrelated.