remove `freightMetadata` two-argument form

[fykaa]

The freightMetadata function currently supports a deprecated two-argument form that needs to be removed in v1.10 as per the deprecation notice.

Current Usage

// Two-form argument (deprecated)
freightMetadata(freightName, 'key-name')

New Usage

// Single-form argument with map access
freightMetadata(freightName)['key-name']

Tasks

• remove the two-argument form support from freightMetadata function in functions.go
• update tests in functions_test.go to remove tests for two-argument form
• ensure documentation is updated to remove mentions of two-argument form
• optional: add migration notes in the v1.10 release notes

Additional Context
The two-argument form has been deprecated since v1.8.0. Users have been encouraged to use the single-argument form with map access syntax instead.

Related Links

• Documentation: https://docs.kargo.io/user-guide/reference-docs/expressions#freightmetadatafreightname
• deprecated in https://github.com/akuity/kargo/pull/4861/files

[github-actions]

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