[v23.1.x] Adjust maximum value for memory_share_for_fetch in MemoryStressTest.test_fetch_with_many_partitions

[vbotbuildovich]

Backport #11533 to branch v23.1.x. Note that this issue was created as a placeholder, since the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v23.1.x
git checkout -b vbotbuildovich/backport-11533-v23.1.x-785
git cherry-pick -x 51b3333a0bedb061307e422a0790bed0ef67f3d0

git push origin vbotbuildovich/backport-11533-v23.1.x-785
gh pr create \
  --title "[v23.1.x] Adjust maximum value for `memory_share_for_fetch` in `MemoryStressTest.test_fetch_with_many_partitions`" \
  --base "v23.1.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-11533-v23.1.x-785" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "dotnwat" \
  --milestone "v23.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/11533 "