fix: Relax static id validation in iterations

[pmdartus]

Details

We decided to validate invalid usage of id attributes in the template early on. The compiler reports an error if a static id attribute is used within an iteration.

<template>
    <template for:each={items} for:item="item">
        <span key={item.key} id="b"></span>
                        <!-- ^ 🚨 Error: Static id values are not allowed in iterators. -->
    </template>
</template>

But until recently, the template compiler ignored the usage of static id attributes on an inline iteration. During the AST template compiler refactor #2518, the logic used to validate the usage of static id attributes in iterations was updated, and now also accounts for inline iterations. The new validation is more correct than the old one, but it also introduced a unexpected breaking change.

<template>
    <div for:each={items} for:item="item" key={item.key}>
        <span id="a"></span>
         <!-- ^ 😱  Ignored prior Summer'22 and reported as error in Summer'22. -->
    </div>
</template>

To revert this breaking change, while keeping the correct validation logic in place this PR downgrade the diagnostic level from Error to Warning.

Does this pull request introduce a breaking change?

• ✅ No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• ⚠️ Yes, it does include an observable change.

See above for more details.

GUS work item

W-11130708

[pmdartus]

@ekashida I saw that you merge this PR directly on the summer22 branch. Any objection to merging it also on master?