fix(signals): Remove the protection for state mutation in dev mode

[rainerhahnekamp]

Applying Object.freeze on the state caused too many issues with edge cases, where mutable changes are necessary.

Therefore, this feature is removed.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

Closes #4683

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

Can it be useful to also mention the revert in the migration guide?

https://github.com/ngrx/platform/blob/main/projects/ngrx.io/content/guide/migration/v19.md#throw-error-in-dev-mode-on-state-mutation

[rainerhahnekamp]

useful to also mention the revert in the migration guide

@timdeschryver I'd just remove the whole section. Devs should always install the latest v19?

[timdeschryver]

Thanks @rainerhahnekamp