Streamline xanchor and yanchor names for center/middle

The 'xanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'left', 'center', 'right']

The 'yanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'top', 'middle', 'bottom']

This is for plotly 6.3.0 trying to use fig.add_annotations() but it likely applies everywhere where xanchor and yanchor are used.

It does make sense that for the extremes of the bounding box left right top bottom, the names are different, but why is 'center' used for x direction and 'middle' for y direction? This distinction seems artifical to me and I wonder why this was chosen?

I propose to streamline the naming conventions, but now for backwards compatibility to at least accept both center/middle for x/yanchor.

Inizia dal punto di ingresso fig.add_annotations() e individua come vengono convalidati i valori di xanchor e yanchor. Verifica se la stessa convalida viene condivisa altrove, quindi esamina i test correlati. L’implementazione dovrebbe includere, ove appropriato, l’accettazione compatibile con le versioni precedenti sia di center sia di middle, mantenendo il funzionamento dei valori esistenti.