[Transform] Improve instructions for obsolete transform configuration warning

[pheyos]

In the 8.0 Upgrade Assistant we can see the following critical error for old transform configurations:

The help text tells the user to use _update or _upgrade to fix this issue, but it would be good to include the full endpoint (/_transform/<transform_id>/_update or /_transform/_upgrade) to give the user a more concrete instruction. It might also be good to mention that /_transform/_upgrade will fix this for all transforms while /_transform/<transform_id>/_update will only operate on a single transform.
Maybe something like this?

Use [/_transform/tf2c/_update] to update the configuration of [tf2c] or [/_transform/_upgrade] to do it for all transforms.

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)

[sophiec20]

Suggest we point them towards using _upgrade.

Use [/_transform/_upgrade] to prepare all transforms for upgrade.

[hendrikmuhs]

It seems we miss what happens if you don't upgrade, I suggest:

This transform configuration uses obsolete syntax which will be unsupported after the next upgrade. Use [/_transform/_upgrade] to prepare all transforms for upgrade.

[przemekwitek]

@pheyos: The messages have been changed in the PRs listed above.

After verification you can close this issue.

[pheyos]

Looks good in 7.17.0 upgrade assistant.