Switch to new Upbound DevEx

[ytsarev]

Description of your changes

Modernizes the configuration to use Upbound's new Developer Experience framework:

• Embedded KCL functions replace patch-and-transform compositions
• Composition & E2E tests using up test run commands
• GitHub workflows updated for up CLI and Upbound Cloud integration
• Comprehensive testing with AWS EKS infrastructure for E2E validation
• Updated documentation following modern Upbound project patterns

The observability functionality remains unchanged - this migration improves development experience, testing capabilities, and maintainability while preserving all monitoring features for Crossplane components.

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Composition Tests

 up test run tests/*
  ✓   Parsing tests
  ✓   Checking dependencies
  ✓   Generating language schemas
  ✓   Building functions
  ✓   Building configuration package
  ✓   Pushing embedded functions to local daemon
  ✓   Assert test-xoss
SUCCESS:
SUCCESS: Tests Summary:
SUCCESS: ------------------
SUCCESS: Total Tests Executed: 1
SUCCESS: Passed tests:         1
SUCCESS: Failed tests:         0

e2e

up test run tests/* --e2e
…
s-cluster   True     True    Deleting
   ├─ Object/podmonitorcrossplane-uptest-observability-oss-cluster                                                                                   -        -       Error: objects.kubernetes.crossplane.io "podmonitorcrossplane-uptest-observability-oss-cluster" not found
   ├─ Object/podmonitorcrossplaneproviders-uptest-observability-oss-cluster                                                                          -        -       Error: objects.kubernetes.crossplane.io "podmonitorcrossplaneproviders-uptest-observability-oss-cluster" not found
   └─ Object/podmonitorcrossplanerbacmanager-uptest-observability-oss-cluster                                                                        -        -       Error: objects.kubernetes.crossplane.io "podmonitorcrossplanerbacmanager-uptest-observability-oss-cluster" not found
    | 15:20:45 | delete | Assert Deletion  | SCRIPT    | LOG   |
        === STDOUT
        oss.observe.platform.upbound.io/uptest-observability condition met
    | 15:20:45 | delete | Assert Deletion  | SCRIPT    | DONE  |
    | 15:20:45 | delete | Assert Deletion  | TRY       | END   |
    | 15:20:45 | delete | @chainsaw        | CLEANUP   | SKIP  |
=== NAME  chainsaw
    | 15:20:45 | chainsaw | @chainsaw | CLEANUP   | SKIP  |
--- PASS: chainsaw (0.00s)
    --- PASS: chainsaw/delete (80.17s)
PASS
2025/06/18 15:20:45 Tests Summary:
2025/06/18 15:20:45 - Passed: 1
2025/06/18 15:20:45 - Failed: 0
2025/06/18 15:20:45 - Skipped: 0
SUCCESS:
SUCCESS: Tests Summary:
SUCCESS: ------------------
SUCCESS: Total Tests Executed: 1
SUCCESS: Passed tests:         1
SUCCESS: Failed tests:         0

[kaessert]

Some nits :)