[Event Grid] Headerless Matcher to not check the dynamic headers

[HarshaNalluru]

Fixes #20343

[browser-tests]     #send (CloudEvent schema)
[browser-tests]       × sends a single event
[browser-tests]         Chrome Headless 93.0.4577.0 (Windows 10)
[browser-tests]       RestError: {"Message":"Unable to find a record for the request POST https://endpoint/api/events?api-version=2018-01-01\r\nHeader differences:\r\n    \u003CContent-Type\u003E values differ, request \u003Capplication/cloudevents-batch\u002Bjson; charset=utf-8\u003E, record \u003Capplication/cloudevents-batch\u002Bjson; charset=UTF-8\u003E\r\nBody differences:\r\n","Status":"NotFound"}
[browser-tests]           at handleErrorResponse$2 (../../core/core-client/src/deserializationPolicy.ts:254:15 <- dist-test/index.browser.js:18197:20)
[browser-tests]           at deserializeResponseBody$2 (../../core/core-client/src/deserializationPolicy.ts:166:4 <- dist-test/index.browser.js:18132:46)

// DIFF
request \u003Capplication/cloudevents-batch\u002Bjson; charset=utf-8\u003E, 
record \u003Capplication/cloudevents-batch\u002Bjson; charset=UTF-8\u003E

Headerless matcher doesn't match the headers in playback.
This should make the CI green.

[ellismg]

Seems reasonable to me. I don't fully understand why the casing differs between the recordings and playback and to be honest I'm slightly confused as to why the uppercase UTF-8 appears in the recordings given that it seems we should be sending the lowercase version given the generated code. But I see how this would fix the issue and I agree the header matching is uninteresting for these test cases.

[HarshaNalluru]

I'm unsure as well about what caused the change.
Previously, the test had the same headers, and my tests migration PR succeeded with playback for the browser test.