refactor(sdk-trace-web): fix eslint warning

```
/home/runner/work/opentelemetry-js/opentelemetry-js/packages/opentelemetry-sdk-trace-web/src/utils.ts
  52:14  warning  Unexpected any. Specify a different type  @typescript-eslint/no-explicit-any
```

The eslint comment wasn't working because it was placed on the
wrong line. However, it also wasn't necessary anymore – `keyof any`
was used to obtain the union of all possible object key types –
`string | number | symbol`. TypeScript now provides an alias for
this in the standard libaray that gives exactly the same result.

Also fixed what appears to be a TypeError on an un-annotated `let`
binding, not sure why it wasn't caught by the type checker, maybe
it was just missed on the version of TS we are using.

Ref open-telemetry#5365

packages/opentelemetry-sdk-trace-web/src/utils.ts

@@ -46,10 +46,9 @@ function getUrlNormalizingAnchor(): HTMLAnchorElement {
  * @param obj
  * @param key
  */
-// eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function hasKey<O extends object>(
   obj: O,
-  key: keyof any
+  key: PropertyKey
 ): key is keyof O {
   return key in obj;
 }
@@ -67,8 +66,8 @@ export function addSpanNetworkEvent(
   entries: PerformanceEntries,
   refPerfName?: string
 ): api.Span | undefined {
-  let perfTime = undefined;
-  let refTime = undefined;
+  let perfTime: number | undefined;
+  let refTime: number | undefined;
   if (
     hasKey(entries, performanceName) &&
     typeof entries[performanceName] === 'number'