Add Last Modified Field For Non-Versioned Items

[Jdubrick]

Please specify the area for this PR

What does does this PR do / why we need it:
This PR adds the logic for setting the lastModified field for stacks/samples that do not have a version. Before this change any samples that did not have a version had the lastModified field set to the default time.Time value. This change allows for the dates to be set properly.

You can reference the conversation around this issue here: #249 (comment)

This PR is flagged as WIP because I believe the changes made to registry here may need to be merged first for tests to pass as it contains changes to how the last_modified.json is generated during build.

Which issue(s) this PR fixes:

Fixes devfile/api#1607

PR acceptance criteria:

• Test Coverage
  • Are your changes sufficiently tested, and are any applicable test cases added or updated to cover your changes?

Documentation (WIP)

• Does the REST API doc need to be updated with your changes?
• Does the registry library doc need to be updated with your changes?

How to test changes / Special notes to the reviewer:
You should be able to test the following:

• go test ./... for index/generator passes
• go test ./... for index/server passes
• Build the registry locally with the test data (bash build_registry.sh) and deploy it.
  • You should be able to see that the samples previously showing the default time are now set

Example of samples being set:

{
   "name": "java-springboot-basic",
   "displayName": "Basic Spring Boot",
   "description": "A simple Hello World Java Spring Boot application using Maven",
   "type": "sample",
   "tags": [
     "Java",
     "Spring"
   ],
   "icon": "https://raw.githubusercontent.com/devfile-samples/devfile-stack-icons/main/spring.svg",
   "projectType": "springboot",
   "language": "java",
   "git": {
     "remotes": {
       "origin": "https://github.com/devfile-samples/devfile-sample-java-springboot-basic.git"
     }
   },
   "lastModified": "2024-02-15T11:45:48+01:00"
 },
 {
   "name": "python-basic",
   "displayName": "Basic Python",
   "description": "A simple Hello World application using Python",
   "type": "sample",
   "tags": [
     "Python"
   ],
   "icon": "https://www.python.org/static/community_logos/python-logo-generic.svg",
   "projectType": "python",
   "language": "python",
   "git": {
     "remotes": {
       "origin": "https://github.com/devfile-samples/devfile-sample-python-basic.git"
     }
   },
   "lastModified": "2024-02-15T11:45:48+01:00"
 }

[thepetk]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Jdubrick, thepetk

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