dep(Canvas#getCenter): migrate to getCenterPoint

[ShaMan123]

getCenter doesn't conform with object's getCenterPoint and it should.
This is non breaking, though I think we should remove getCenter right away.

[github-actions]

Code Coverage Summary

> fabric@5.1.0 coverage:report
> nyc report --reporter=lcov --reporter=text

-----------|---------|----------|---------|---------|-----------------------------------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-----------|---------|----------|---------|---------|-----------------------------------------------
All files  |   83.27 |    76.71 |   86.45 |   82.99 |                                               
 fabric.js |   83.27 |    76.71 |   86.45 |   82.99 | ...,29666,29791,29871-29936,30059,30158,30375 
-----------|---------|----------|---------|---------|-----------------------------------------------

[asturur]

This is probably my fault.
Initially my first js days i thought creating an object was better than getting a new point instance.
To be honest i don't thing there is much difference, and if there is, we have to stick with it for consistency sake

[github-actions]

Code Coverage Summary

> fabric@5.1.0 coverage:report
> nyc report --reporter=lcov --reporter=text

-----------|---------|----------|---------|---------|-----------------------------------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-----------|---------|----------|---------|---------|-----------------------------------------------
All files  |   83.28 |    76.71 |   86.52 |      83 |                                               
 fabric.js |   83.28 |    76.71 |   86.52 |      83 | ...,29666,29791,29871-29936,30059,30158,30375 
-----------|---------|----------|---------|---------|-----------------------------------------------