Fix parsing ABIs with multiple l1_handler entries

[ddoktorski]

Closes #1296

Introduced changes

• Fix bug with parsing an ABI with multiple l1_handler entries
• Update docs before 0.20.0 release

• This PR contains breaking changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.01%. Comparing base (167e873) to head (83162a7).

Additional details and impacted files

@@               Coverage Diff               @@
##           development    #1302      +/-   ##
===============================================
+ Coverage        97.99%   98.01%   +0.02%     
===============================================
  Files               93       93              
  Lines             4582     4580       -2     
===============================================
- Hits              4490     4489       -1     
+ Misses              92       91       -1     

Files	Coverage Δ
starknet_py/abi/v2/model.py	100.00% <ø> (ø)
starknet_py/abi/v2/parser.py	94.33% <100.00%> (+0.82%)	⬆️
starknet_py/contract.py	97.68% <100.00%> (ø)

[ddoktorski]

I would prefer to rename it to l1_handlers but this is a bit problematic given that in some places we use Union[AbiV0, AbiV2], and AbiV0 contains l1_handler, so I decided to retain the same name here.