Change description of destination_project field in backup and restore channel resources.

[aru31]

This PR updates comments for destination_project field for backup and restore channel.
Fixes b/417086441

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 4 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 4 files changed, 4 insertions(+), 4 deletions(-))

[modular-magician]

Tests analytics

Total tests: 28
Passed tests: 28
Skipped tests: 0
Affected tests: 0

Click here to see the affected service packages

• gkebackup

🟢 All tests passed!

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