Update InstallationStore design to use two callbacks instead of four

[stevengill]

Description

With the introduction of Org Wide Apps, we introduced two new methods to our @slack/oauth installationStore to support this. fetchOrgInstall & saveOrgInstall were added to the installationStore definition, which already included fetchInstall & saveInstall.

With us coming up to releasing a new major, now is a good time to revisit this design and bring it in line with our other libraries that only support the original two callbacks in installationStore for both single workspace and org wide app installs. So we want to drop support for fetchOrgInstall and saveOrgInstall.

This would be a breaking change.

What type of issue is this? (place an x in one of the [ ])

• enhancement (feature request)

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

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Packages:

• @slack/oauth