Update README.md to reflect Cloud Run functions rebrand

[jama22]

Description

Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[code-review-assist-experimental]

Hi @jama22 and reviewers,

I'm currently reviewing this pull request and will post my detailed review in a few minutes. In the meantime, here's a quick summary to help everyone get up to speed:

This pull request updates the README.md file in the functions/ directory to reflect the rebranding of Cloud Functions to Cloud Run functions. The main changes are:

• Updated README introduction: The introduction now accurately reflects the new name and provides a clear explanation of the two versions of Cloud Run functions (1st gen and 2nd gen, formerly known as Cloud Functions 1st and 2nd gen). This includes links to the relevant documentation.
• Updated sample list: The list of samples in the README has been updated to reflect the new structure and location of some samples. Specifically, the Cloud Pub/Sub sample has been moved to the v2/ directory.

The changes are concise and well-documented, making the README easier to understand for users. The updated README accurately reflects the current state of Cloud Run functions and their associated samples.

---

And now, a little haiku to lighten the mood:

Cloud Run's new name,
Docs updated, clear and bright,
Code flows smoothly on.

Looking forward to a thorough review!