upgrades: run upgrade steps for final release when upgrading to an alpha or beta release

This allows definition and testing of upgrade steps as we work towards
a stable release.
upgrades: target 1.21 steps at 1.21 final, not alpha1

Targeting at alpha1 was just a temporary step until the upgrade step
selection logic was changed. Also added a check to ensure that upgrade
steps are only targeted to final releases.
cmd/jujud: test fix due to changed upgrade steps selection logic
Merge pull request #1010 from mjs/upgrade-steps-selection

Tweak upgrade steps selection logic and retarget 1.21 upgrade steps

upgrades: run upgrade steps for final release when upgrading to an alpha or beta release

This allows definition and testing of upgrade steps as we work towards a stable release.

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upgrades: target 1.21 steps at 1.21 final, not alpha1

Targeting at alpha1 was just a temporary step until the upgrade step selection logic was changed. Also added a check to ensure that upgrade steps are only targeted to final releases.