Promoting enable_emergent_maintenance to GA and setting to optional as this is not required

[NA2047]

This is supported in the update and does not require a recreation in gcloud

https://docs.cloud.google.com/compute/docs/reference/rest/v1/reservations/update

compute: added `enable_emergent_maintenance` to `resource_compute_reservation` (GA)

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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[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, 31 insertions(+), 5 deletions(-))
google-beta provider: Diff ( 2 files changed, 11 insertions(+), 2 deletions(-))
terraform-google-conversion: Diff ( 4 files changed, 21 insertions(+))

[modular-magician]

Tests analytics

Total tests: 1403
Passed tests: 1313
Skipped tests: 90
Affected tests: 0

Click here to see the affected service packages

• compute

🟢 All tests passed!

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[github-actions]

@malhotrasagar2212 This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[github-actions]

@GoogleCloudPlatform/terraform-team @malhotrasagar2212 This PR has been waiting for review for 1 week. Please take a look! Use the label disable-review-reminders to disable these notifications.

[shuyama1]

@modular-magician reassign-reviewer shuyama1

[shuyama1]

@NA2047 I do see this field in the test mmv1/templates/terraform/examples/reservation_basic_beta.tf.tmpl. Would it be possible to promote that test (if it doesn't contain other beta-only field) and also add an update test for it? Thanks!