chore: update version for 1.18 rc

[sicoyle]

Description

Update VERSION file to be the 1.18 RC

after this i will run

git checkout release-1.18
git pull upstream release-1.18
git tag v1.18.0rc0
git push upstream v1.18.0rc0
Tag push triggers the dapr-python-release workflow which publishes all 7 packages to PyPI.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #[issue number]

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[Copilot]

Pull request overview

Updates the repository’s single source-of-truth VERSION file to a 1.18 release-candidate value, which is consumed by all packages’ build metadata.

Changes:

• Bump VERSION from 1.18.0.dev to 1.18.0rc0.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-1.18@8baa14b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff               @@
##             release-1.18    #1039   +/-   ##
===============================================
  Coverage                ?   82.46%           
===============================================
  Files                   ?      146           
  Lines                   ?    14501           
  Branches                ?        0           
===============================================
  Hits                    ?    11958           
  Misses                  ?     2543           
  Partials                ?        0           

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