Use einsum_v2 instead of einsum_v1 for Tensorflow

[rmoyard]

Context:

tf.einsum is currently replace by its version einsum_v1 for compatibility with TensorFlow 2.1.~

Description of the Change:

Remove the restriction and use tf.einsum.

Benefits:

Support new tf.einsum functionnalities.

Possible Drawbacks:

PennyLane does not support TensorFlow 2.1.~ anymore

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2683 (cc3cdf2) into master (bee93e4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2683   +/-   ##
=======================================
  Coverage   99.59%   99.60%           
=======================================
  Files         249      249           
  Lines       20280    20275    -5     
=======================================
- Hits        20197    20194    -3     
+ Misses         83       81    -2     

Impacted Files	Coverage Δ
pennylane/devices/default_qubit_tf.py	95.31% <ø> (+2.55%)	⬆️

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[dime10]

Looks good 👍