Update bootstrap URLs quickstart.rst

[jpoikela]

use GitHub URLs to download bootstrap files

What does this PR do?

change URLs to current download location of bootstrap script and sha

What issues does this PR fix or reference?

Fixes "masterless" quickstart failure caused by deprecated URLs.

Previous Behavior

Tried to download script from https://bootstrap.saltproject.io and failed.

New Behavior

Downloads script from https://github.com/saltstack/salt-bootstrap/releases/latest/download/bootstrap-salt.sh (and .sha256)

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [ ] Tests written/updated

Commits signed with GPG?

No

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[welcome[bot]]

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