fix(install): allow "minimumDependencyAge" object without "age" (#34523)

Previously, specifying `minimumDependencyAge` as an object in
`deno.json` required an `age` field, so a config like
`{ "minimumDependencyAge": { "exclude": ["npm:chalk"] } }` failed with
`missing field \`age\``. This made it impossible to keep the exclude
list in the config file while supplying the age via the
`--minimum-dependency-age` flag. The field is now optional, falling back
to no age constraint when omitted.

Fixes #34521