Add release notes for the new connection drain (+ use dashes instead of underscore)

[frouioui]

Description

This PR adds the release notes for the changes implemented by #16298.

It also changes the flag to use dashes instead of underscores.

Related Issue(s)

• Related to RFC: Drain MySQL connection in VTGate upon termination of VTGate #15971

[vitess-bot]

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• Ensure that the Pull Request has a descriptive title.
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.70%. Comparing base (d236ccd) to head (c9d4b2c).
Report is 12 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/plugin_mysql_server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16334      +/-   ##
==========================================
- Coverage   68.72%   68.70%   -0.03%     
==========================================
  Files        1547     1547              
  Lines      198290   198287       -3     
==========================================
- Hits       136279   136229      -50     
- Misses      62011    62058      +47     

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[mattlord]

Had some suggestions, but up to you, whatever you prefer.

[mattlord]

Suggested change

	We are adding a new way to terminate VTGate in v21 by using a connection drain timeout rather than an activity drain timeout.
	The goal of this new drain is to disallow new connections when VTGate is shutting down, but keep allowing idle/actives
	connections to execute queries until they disconnect or until the `--onterm_timeout` is reached.
	We added a new option to affect the VTGate shutdown process in v21 by using a connection drain timeout rather than the older activity drain timeout. The goal of this new connection draining option is to disallow new connections when VTGate is shutting down, but continue allowing existing connections to finish their in flight work, giving them up to `--onterm_timeout` to complete it.
	This new behavior can be enabled by specifying the new `--mysql-server-drain-onterm` flag to VTGate.

[frouioui]

I did not follow the suggested text at 100%, but I improved it via 325c619, here is what I wrote:

We added a new option to affect the VTGate shutdown process in v21 by using a connection drain timeout rather than the older activity drain timeout.
The goal of this new behavior, connection draining option, is to disallow new connections when VTGate is shutting down,
but continue allowing existing connections to finish their work until they manually disconnect or until the `--onterm_timeout` timeout is reached,
without getting a `Server shutdown in progress` error.

This new behavior can be enabled by specifying the new `--mysql-server-drain-onterm` flag to VTGate.

[deepthi]

I hope you got all references to the underscore version :)

[frouioui]

I hope you got all references to the underscore version :)

The release notes + go/test/endtoend/vtgate/connectiondrain/main_test.go are the only places where we use/set the flag.