Remove explicit enable command

[vishnoianil]

Remove explicit enable command, only maintainer teams member will be able to run commands.

Bot will post a welcome message when required labels are set on the PR (currently set to skill and knowledge).

Our bot command name is not changed to @instructlab-bot from @instruct-lab-bot, due to renaming at the instruct lab organization level. But we do support the legacy bot username but post a deprecated warning on the PR.

Fixes #253 Fixes #261

This PR https://github.com/vishnoianil/taxonomy/pull/23 shows the workflow.

[vishnoianil]

Some snapshot of the new changes :

• enable command is gone. now when maintainer add any of the label (mentioned in the required_labels, such skill or knowledge) it posts the welcome message, also a readiness check. It only happens whenever the first required label is set, just to make sure that bot doesn't post on every required label if there are more than one. If PR is being reset and labels are removed and readded, it skip posting as well.

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• If user fire "enable" command, bot is going to post deprecated warning as well.

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• We now support @Instructlab-bot command and deprecated the old @instruct-lab-bot command. If user uses old command it posts deprecated command warning. Business as usual with new command.

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• Now bot allows only the members of the maintainer teams (set through maintainers config of bot) to run precheck/generate/generate-local.

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