build: upgrade to fmt v9

[dotnwat]

This unlocks clang-18.

Backports Required

• [x] 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
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• none

[dotnwat]

@rockwotj mind testing out clang-18 locally? i don't have a good way to turn on clang-18 in CI other than changing the default in .bazelrc.

if that's working ok for you, then i think we can do the pgo/lto build and then switch over to clang-18 as the default?

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55083#019227bd-54ca-457f-8497-a9ba09f3e5f0

[rockwotj]

Note that bazel build --config=clang-18 //... will auto-download clang-18 for you. But yeah I'm running it locally

[rockwotj]

Tree builds with clang-18 just fine. I'll give 19 a shot too 😄

[dotnwat]

Note that bazel build --config=clang-18 //... will auto-download clang-18 for you. But yeah I'm running it locally

yeh, that's what i'm doing locally. i just meant i can't actually make that happen in CI in the context of this PR.

[dotnwat]

sweet, then i think we can merge this? i tested the open source cmake build, so we probably just need to bump 3rdparty.cmake and it should work out of the box

[rockwotj]

Yeah let's :shipit: