admin: fix copy paste mistake leading to incorrect value for max_collectible_offset

[nvartolomei]

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [x] v24.2.x
• [x] v24.1.x
• [ ] v23.3.x

Release Notes

• none

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55039#01922519-1240-4de5-b7d9-5e56843e6f3e

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55111#019229d7-8b41-49e3-b443-3bdc86c5774c

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55111#019229d7-8b39-431f-bb9c-566768304a3c

[nvartolomei]

@dotnwat I can't find any use cases beyond debug endpoint/debug bundle. Updated commit message to say that.

[vbotbuildovich]

/backport v24.2.x

[vbotbuildovich]

/backport v24.1.x