Annual conversion in .grain() not working for pandas<2.0.0 [BUG]

[cdietrich215]

Describe the bug
Getting an error when converting to an annual grain when pandas <2.0.0 using the .grain() method. I believe this was caused by the release of chainladder version 0.8.21

To Reproduce

The following code causes an error when chainladder verion = 0.8.23 and pandas version = 1.5.2

import chainladder as cl
import pandas as pd

tri = cl.load_sample('prism')['Incurred'].sum()
tri.grain('OYDY')

[Evgeny-Avdeev]

I think mistake is in triangle.py line 693.
Now it looks like:
freq = {"Y": "Y", "S": "2Q"}.get(ograin_new, ograin_new)
But should be as it was before (ver. 0.8.15):
freq = {"Y": "A", "S": "2Q"}.get(ograin_new, ograin_new)