packer fails to validate `deprecate_at` unless it's a literal string

#### Overview of the Issue

I am using the `amazon-ebs` builder with the `amazon-ami` data source, to build a custom AMI based off an existing upstream AMI.

I've found that I can't run `packer validate` if I set `deprecate_at` to anything other than a literal string. My upstream AMI is replaced on a schedule, so I wanted to set `deprecate_at` to e.g. `timeadd(data.amazon-ami.source.creation_date, duration)`. But when I do, I get an error like the following:

```
Error: 1 error(s) occurred:

* deprecate_at is not a valid time. Expect time format: YYYY-MM-DDTHH:MM:SSZ
```

I think this is a bug:

- If I cause the result to be written to a file instead, I see that the written date/time matches the expected format.
- If I replace `timeadd(...)` with the value written to the file, it validates.
- If I replace `timeadd(data.amazon-ami...)` with `timeadd(timestamp(), ...)`, it validates.
- If I just run `packer build example.pkr.hcl` it works fine, setting the expected deprecation time.

This blocks me because my CI routine runs `packer validate` on the template, so even though `packer build` would work, CI fails. I would have to work around it by calculating the deprecation_time outside Packer and passing the result in as a variable.

Maybe this is a Packer problem? The data source is not evaluated by `packer validate`, and the parameter expects a particularly-formatted string, so there would be no way to ensure a calculated value is valid. I wonder if, instead of failing to validate, this could issue a warning instead?

#### Reproduction Steps

See gist at https://gist.github.com/saxonww/d237f1ff2a3292cf916b02d85eca8cb5

### Plugin and Packer version

Packer v1.10.1

Plugin version v1.3.2

### Simplified Packer Buildfile

See gist at https://gist.github.com/saxonww/d237f1ff2a3292cf916b02d85eca8cb5

### Operating system and Environment details

A Docker container based on the following:

```
$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
```

Architecture is x86_64.

### Log Fragments and crash.log files

Setting PACKER_LOG=1 doesn't yield anything useful.