Add migration guide for google_firebaserules_release breaking changes

[rainshen49]

To maintainers: please add c2thorn as a reviewer because he is coordinating other parts of this change with DCL.

Fixes hashicorp/terraform-provider-google#13235

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).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

docs: added the breaking change of the resource `google_firebaserules_release` to the 5.0.0 guides

[modular-magician]

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@zli82016, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

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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.

Terraform GA: Diff ( 1 file changed, 54 insertions(+))
Terraform Beta: Diff ( 1 file changed, 54 insertions(+))

[zli82016]

@rainshen49 , should @c2thorn be added as the reviewer? Thanks.

[rainshen49]

@zli82016 Yes. I don't have the permission to configure my own reviewers.

[zli82016]

Added @c2thorn as the reviewer.

[c2thorn]

lgtm

[c2thorn]

Hi, @rainshen49, I created hashicorp/terraform-provider-google#15497 to track the DCL change