Replace Fastly VCL cstr_escape w/ json.escape

[akesle]

Purpose of this pull request

Bring Sumo Logic's Fastly integration VCL examples in line with Fastly's own documentation for integrating with Sumo Logic. This concerns the preferred use of the VCL json.escape function over the somewhat ineffective (in the context of JSON) use of the suggested cstr_escape.

TL:DR Using cstr_escape in a Fastly VCL log format string may still result in invalid JSON sequences, whereas substituting json.escape will alleviate all those issues.

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

Originated by Fastly.

[cla-bot]

Thank you for your contribution! As this is an open source project, we require contributors to sign our Contributor License Agreement and do not have yours on file. To proceed with your PR, please sign your name here and we will add you to our approved list of contributors.

[akesle]

Thank you for your contribution! As this is an open source project, we require contributors to sign our Contributor License Agreement and do not have yours on file. To proceed with your PR, please sign your name here and we will add you to our approved list of contributors.

I have signed this license agreement.

[jpipkin1]

Approved, @akesle.

[akesle]

Approved, @akesle.

Much appreciated, @jpipkin1 👋