feat: cut release 3.5.0-gmp (rc.1 demo)

[bwplotka]

This is a starting point for 3.5.0 fork release without the need of any work on prometheus-engine (go/gmp:fork-toil)

This is to show how easier it is to sync the fork using new scripts (go/gmp:toil-automation) via:

SOURCE_BRANCH=bwplotka/cut-release-2.53.5-gmp UPSTREAM_TAG=v3.5.0-rc.1 CHECKOUT_DIR=~/Repos/tmp-release2 hack/release-forksync.sh

Took me 20m to run the script and fix all the immediate git conflicts. Ofc there's still more work to get it buildable and tested (e.g. go-kit/log was replaced with slog, so there's some work on export package), but it's considerably easier, more consistent and faster to sync, even acros MAJOR prometheus version.

The 3.5.0 is the LTS release and is still in progress upstream (RC.1 only available that's why pointing to rc.1). Once there is a final version and we fix this release-3.5.0-rc.1-gmp branch, it will be as easy as running, likely with fully ready code branches to relase.

SOURCE_BRANCH=release-3.5.0-rc.1-gmp UPSTREAM_TAG=v3.5.0 CHECKOUT_DIR=~/Repos/tmp-release2 hack/release-forksync.sh

cc @pintohutch @bernot-dev

[pintohutch]

Nice! You should walk us through the scripts at some point :)