BUG: SeriesGroupBy works with any column name specified in NamedAgg
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python3
s = pd.Series([1,1,2,2,3,3,4,5])

s.groupby(s.values).agg(one = pd.NamedAgg(column='anything',aggfunc='sum'))
   one
1    2
2    4
3    6
4    4
5    5

s.groupby(s.values).agg(one=('something','sum'))

   one
1    2
2    4
3    6
4    4
5    5
```

#### Problem description

For `SeriesGroupBy.agg` named aggregation, it accepts any column name as mentioned in the above examples but Series doesn't have any columns.

#### Expected Output
After discussing with @TomAugspurger and @MarcoGorelli in another issue #34380 regarding this issue in the comments, We came to a solution that 
> So for SeriesGroupBy.agg, if there are any tuples or NamedAgg present in kwargs then I think we should raise.
Disallowing `NamedAgg` and tuples with `SeriesGroupBy`.
```python3
s.groupby(s.values).agg(one = pd.NamedAgg(column='anything',aggfunc='sum'))
# Error should be raised.
s.groupby(s.values).agg(one=('something','sum'))
# Error should be raised.
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.3
numpy            : 1.18.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 46.1.3
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
