refactor(services/webdav): deprecate enable_conditional_read

[YuangGao]

Which issue does this PR close?

Refs #6708.

Rationale for this change

enable_conditional_read only masks capabilities; the request path never reads it. RFC-6707 moves endpoint-specific capability selection to CapabilityOverrideLayer, which #7562 already applied to disable_copy and enable_user_metadata.

What changes are included in this PR?

• Deprecate enable_conditional_read as a no-op shim.
• Declare the four read_with_if_* capabilities unconditionally.
• Migrate the four nginx CI setups to OPENDAL_TEST_CAPABILITY_OVERRIDES.

Verified on nginx-dav and Nextcloud: 127 behavior tests pass on both.

Are there any user-facing changes?

enable_conditional_read no longer affects capabilities. Use CapabilityOverrideLayer to disable the four read_with_if_* capabilities for endpoints without ETag support.

Breaking changes

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