fix: OPTIC-1045: Update default placeholder text used in task data to be more clear

[bmartel]

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

The default placeholder text used with our Labelling configs in the Preview are often distracting and not clearly obvious that it is just sample data. This updates those which were containing non-obvious text to be clearly sample text.

Does this PR introduce a breaking change?

(check only one)

• Yes, and covered entirely by feature flag(s)
• Yes, and covered partially by feature flag(s)
• No
• Not sure (briefly explain the situation below)

Which logical domain(s) does this change affect?

Label Config Preview, Sample Task Data

[netlify]

✅ Deploy Preview for heartex-docs canceled.

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[netlify]

✅ Deploy Preview for label-studio-docs-new-theme canceled.

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🔨 Latest commit	fa03891
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[MihajloHoma]

/git merge

Workflow run
Successfully merged: create mode 100644 web/libs/ui/src/lib/Toast/Toast.tsx

[robot-ci-heartex]

Follow Merge downstream workflow has been failed.

Workflow run

[bmartel]

/git merge develop

Workflow run
Successfully merged: 6 files changed, 9 insertions(+), 9 deletions(-)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.76%. Comparing base (fc49438) to head (fa03891).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6656   +/-   ##
========================================
  Coverage    76.76%   76.76%           
========================================
  Files          169      169           
  Lines        13865    13865           
========================================
  Hits         10644    10644           
  Misses        3221     3221           

Flag	Coverage Δ
pytests	76.76% <ø> (ø)

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