fix(admin): outdated master key warning check

[kon14]

This fixes Admin's unset MASTER_KEY env var security warning check not working for new env name.
I kept the older form (masterkey) around.

We can cleanup older env names in v0.15 with the next compak break.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other (please describe)

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the main branch
• When resolving a specific issue, it's referenced in the PR's description (e.g. fix #xxx, where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature

Other information: