Remove auto-updating feature from sfdx

[AndrewRayCode]

Is your feature request related to a problem? Please describe.
There are occasional issues with sfdx that appear without warning, for example #684. These issues affect users locally as well as CI/CD systems that use sfdx.

What are you trying to do
Not have to worry about the sfdx version self upgrading and spontaneously breaking something.

Describe the solution you'd like
sfdx to no longer update and make it clear which version it's on. Right now it's confusing because there seem to be core specific libraries that have their own versions, not a clear overall sfdx version.

Describe alternatives you've considered
Trying to force lock in a version with reverse engineering the core packages and running specific install commands. Doesn't feel like it's worth it.

Additional context
Auto updating doesn't seem appropriate for a CLI tool. It introduces nondeterministic software behavior, and removes the user's ability to triage bugs by downgrading tool versions.

[github-actions]

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[amphro]

You can already turn off auto-updates.

1. If using the installers, set the environment variable: SFDX_DISABLE_AUTOUPDATE=true (doc'ed here)
2. If using npm, it doesn't auto-update by default. i.e. npm install sfdx-cli --global. You can also hardcode to a specific version: npm install sfdx-cli@49 --global

If you want CLI auto-updates but want to prevent certain commands from updating, you can lock to specific versions. sfdx plugins:install [email protected].