make updating config.software_config.image_version beta-only

[megan07]

Fixes hashicorp/terraform-provider-google#11300

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

composer: made the `google_composer_environment.config.software_config.image_version` field immutable as this functionality is only available in beta.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 15 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[megan07]

Did you test setting these in tandem to ensure this is possible?

Was this comment meant for the other PR? If so, yes. If it's meant for this one, I'm not sure which these you're referring to :)

[ScottSuarez]

Sorry, comment was for the other pr haha