refactor(flask): replace SpanAttributes with semconv attributes

[artemziborev]

Description

This PR replaces deprecated usage of SpanAttributes with semantic convention attributes from opentelemetry.semconv.attributes in the opentelemetry-instrumentation-flask package.

These changes bring the Flask instrumentation module in line with current OpenTelemetry conventions and help modernize the codebase as part of the broader refactoring tracked in the issue below.

Refs #3475

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Ran pytest for instrumentation/opentelemetry-instrumentation-flask/tests/
• Verified that no references to SpanAttributes remain
• Confirmed proper attribute output using updated semconv references

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• [X ] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Unit tests pass for this module

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: artemziborev / name: Artem Ziborev (7fbec0f, a2fa5b0, f1dda55, b93c547, efd599f, a02fa8f, 85df3ae)
• ✅ login: xrmx / name: Riccardo Magliocchetti (176df0e)

[artemziborev]

Hi 👋 Just wanted to kindly follow up.

This PR replaces deprecated SpanAttributes usage in the Flask instrumentation with the recommended semantic convention attributes (http.method, http.route, etc.), as described in #3475.

All changes were tested locally and verified against the existing test suite. Please let me know if anything should be adjusted or improved — happy to iterate!

Thanks for your time and for maintaining this project 🙏

[artemziborev]

Hi! All checks have passed — would love to get a review when you have a moment. Let me know if any changes are needed 🙌