Feature Request: deprecate the `--legacy-replication-lag-algorithm` VTGate flag

[timvaillancourt]

Feature Description

As a follow-up to #5251, where a simpler algorithm for lag detection was added to VTGate, this issue proposes the full deprecation of the --legacy-replication-lag-algorithm flag, which seems to be the original intent of #5251 about 6 years ago

--legacy-replication-lag-algorithm=false is currently in use at some of the largest Vitess deployments (5 years+ at Slack) and has been proven to be more stable. There is also a desire from the maintainers to remove the old code

This deprecation must happen in 3 x phases:

1. v24 - change default to false, warn about future deprecation in changelog
   • PR: vtgate: default to --legacy-replication-lag-algorithm=false #18915
2. v25 - deprecate the flag, make it cause a no-op
   • PR: TBD
3. v26 - remove the flag 🧹
   • PR: TBD

Use Case(s)

General users of VTGate