Deprecate twopc_enable flag

[harshit-gangal]

Description

This PR deprecate twopc_enable flag to enable TwoPC.
The twopc_abandonage is still mandatory to be set on VTTablet for TwoPC transactions prepare to work.

Related Issue(s)

• RFC: Atomic Distributed Transactions #16245

Checklist

• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.39%. Comparing base (0b51839) to head (db4e613).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17279      +/-   ##
==========================================
+ Coverage   67.37%   67.39%   +0.02%     
==========================================
  Files        1573     1573              
  Lines      253113   253129      +16     
==========================================
+ Hits       170538   170601      +63     
+ Misses      82575    82528      -47     

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[deepthi]

Are we really determining whether to allow 2PC based on twopc_abandonage? Ideally it should be controlled solely by transaction_mode on vtgate, and vttablets should be able to participate.
My suggestion would be to pick a reasonable default for twopc_abandonage and use that. Also we should be changing that to a Duration field instead of float which I assume is being interpreted as seconds.