Fixup envoy admin port constants

[bleggett]

Do you have a suggestion for code improvement or tracking existing technical debt? Please describe.

Spun out of solo-io#10239

That PR removed envoyutil.admincli.DefaultAdminPort in favor of defaults.EnvoyAdminPort - they are the same value and having 2x constants in the codebase is confusing - most tests were using the latter already.

That being said

1. We should try and move the constants closer to where they're used, if we can (should be done in one PR).
2. I believe gloo admin port has the same problem, and should also be fixed.

Describe the solution you'd like

No response

Additional Context

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