fix: use debug level for readiness checks logs

[czeslavo]

What this PR does / why we need it:

It turns out s.logger.V(util.DebugLevel).Error(err, "...") doesn't result in a log message with a debug, but an error level. The PR fixes logs that were intended to use debug-level with the error as a log field.

Which issue this PR fixes:

Customer reported

{"error":"making HTTP request: Get \"https://172.17.1.180:8444/status\": context deadline exceeded","level":"error","logger":"setup.readiness-checker","msg":"already created client for \"https://172.17.1.180:8444/\" is not ready, moving to pending","time":"2023-10-31T09:52:17Z"}

kind of logs presence repeating quite often. Error logs should not be used for that kind of message as they're signaling expected situations that users should not be worried about.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[programmer04]

It's exactly like you described, calling Error always overwrites the level to error

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (10879ed) 75.4% compared to head (9f2b6c0) 75.2%.

❗ Current head 9f2b6c0 differs from pull request most recent head 9b4e3a0. Consider uploading reports for the commit 9b4e3a0 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5029     +/-   ##
=======================================
- Coverage   75.4%   75.2%   -0.3%     
=======================================
  Files        167     167             
  Lines      18796   18796             
=======================================
- Hits       14184   14144     -40     
- Misses      3787    3830     +43     
+ Partials     825     822      -3     

Files	Coverage Δ
internal/clients/readiness.go	89.8% <100.0%> (ø)
internal/dataplane/sendconfig/backoff_strategy.go	84.6% <100.0%> (ø)

... and 10 files with indirect coverage changes

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[github-actions]

The backport to release/2.12.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/2.12.x release/2.12.x
# Navigate to the new working tree
cd .worktrees/backport-release/2.12.x
# Create a new branch
git switch --create backport-5029-to-release/2.12.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 afe12ed7a15299de3343132002c2451fe8aa42cb
# Push it to GitHub
git push --set-upstream origin backport-5029-to-release/2.12.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/2.12.x

Then, create a pull request where the base branch is release/2.12.x and the compare/head branch is backport-5029-to-release/2.12.x.