Update spectrum class name from Spectrum1D to Spectrum

[rosteen]

Does what it says on the tin - based on the result of https://github.com/astropy/specutils/discussions/1087. Spectrum1D will raise a deprecation warning when initialized for some deprecation period.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.80645% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 72.72%. Comparing base (4041081) to head (40ac61b). Report is 37 commits behind head on v2.0-dev.

Files	Patch %	Lines
specutils/io/default_loaders/apogee.py	25.00%	3 Missing :warning:
specutils/io/default_loaders/sdss.py	25.00%	3 Missing :warning:
specutils/io/default_loaders/sixdfgs_reader.py	25.00%	3 Missing :warning:
specutils/spectra/spectrum.py	75.00%	3 Missing :warning:
specutils/io/default_loaders/aaomega_2df.py	33.33%	2 Missing :warning:
specutils/io/default_loaders/twodfgrs_reader.py	33.33%	2 Missing :warning:
specutils/io/default_loaders/wcs_fits.py	71.42%	2 Missing :warning:
specutils/manipulation/smoothing.py	60.00%	2 Missing :warning:
specutils/manipulation/utils.py	75.00%	2 Missing :warning:
specutils/io/default_loaders/dc_common.py	50.00%	1 Missing :warning:
... and 7 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v2.0-dev #1126 +/- ## ============================================ + Coverage 71.26% 72.72% +1.45% ============================================ Files 64 62 -2 Lines 4542 4487 -55 ============================================ + Hits 3237 3263 +26 + Misses 1305 1224 -81 ```

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[rosteen]

Looks like the test failure is unrelated, I wonder if that MANGA file was removed or changed url or something...

[rosteen]

This is probably the most important part if anyone reviews:

@deprecated(since="2.0", alternative="Spectrum")
class Spectrum1D(Spectrum):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)