fix(manager/helm-requirements): ensure currentValue is a string

[secustor]

Changes:

Convert currentValue to string if currentValue is defined.

Context:

If versions like 1.4 are defined helm-requirements js-yaml parse them as number and not as string
Prevents:

       "err": {
         "message": "constraint?.startsWith is not a function",
         "stack": "TypeError: constraint?.startsWith is not a function\n    at Object.isSingleVersion (/usr/src/app/node_modules/renovate/lib/versioning/npm/index.ts:39:18)\n    at lookupUpdates (/usr/src/app/node_modules/renovate/lib/workers/repository/process/lookup/index.ts:318:43)\n    at fetchDepUpdates (/usr/src/app/node_modules/renovate/lib/workers/repository/process/fetch.ts:37:32)\n    at /usr/src/app/node_modules/p-map/index.js:57:22"
       }

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 31.66.2 🎉

The release is available on:

• GitHub release
• 31.66.2

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