chore: switch from dependabot to renovate

[yamaceay]

This PR aims to switch from Dependabot to Renovate to ping new dependency updates. Done in two parts: (1) Create renovate config, (2) Delete existing Dependabot workflow

Open questions remain such as but not limited to: (1) How frequently shall we receive the dependency updates?
(2) Is chore(deps) good to go as a prefix or fix(deps) shall be included too?

[yamaceay]

Command:

RENOVATE_CONFIG_FILE=/path/to/ai-sdk-python/renovate.json LOG_LEVEL=debug  npx --yes  renovate --token "$(gh auth token)" --dry-run SAP/ai-sdk-python

Outputs:

DEBUG: Found 17 package file(s) (repository=SAP/ai-sdk-python, baseBranch=main)
 INFO: Dependency extraction complete (repository=SAP/ai-sdk-python, baseBranch=main)
       "stats": {
         "managers": {
           "github-actions": {"fileCount": 13, "depCount": 49},
           "pep621": {"fileCount": 4, "depCount": 56}
         },
         "total": {"fileCount": 17, "depCount": 105}
       }
...
DEBUG: Renovate repository PR statistics (repository=SAP/ai-sdk-python)
       "stats": {"total": 2, "open": 2, "closed": 0, "merged": 0}
DEBUG: Repository result: undefined, status: onboarding, enabled: true, onboarded: false (repository=SAP/ai-sdk-python)
DEBUG: Repository timing splits (milliseconds) (repository=SAP/ai-sdk-python)
       "splits": {
         "init": 18898,
         "onboarding": 1317,
         "extract": 3504,
         "lookup": 3911,
         "update": 0
       },
       "total": 27964
DEBUG: Package cache statistics (repository=SAP/ai-sdk-python)
       "get": {"count": 81, "avgMs": 362, "medianMs": 352, "maxMs": 1068, "totalMs": 29344},
       "set": {"count": 0, "avgMs": 0, "medianMs": 0, "maxMs": 0, "totalMs": 0}