backport/vscode/1.50: chore(telemetry): remove legacy back-compat (#6265)

[bobheadxi]

When V2 telemetry first launched, we had to be careful to support older instances with V1 event logs, and shortly after the launch there were a series of bug fixes that necessitated several compatibility modes from 5.2.0 to 5.2.5, which was released in December 2023. After 5.2.5, we never committed to removing this back-compat as we waited for customers to slowly upgrade past 5.2.5.

Sadly, this came back to haunt us when 5.10.0 came out a few days ago, and our compatibility mode detector sorted 5.10.0 < 5.2.5, causing V2T to be exported as old event logs, which prevented them from being exported. After finding the bug and discussing it a bit, we've decided to remove this back-compat
entirely since most Cody customers have upgraded past 5.2.5. This simplifies the code significantly.

With the backcompat gone, this basically removes the need for any of the old log-event-to-dotcom stuff, which allowed for the removal for quite a lot of unused code, which simplifies even more stuff.

Closes
https://linear.app/sourcegraph/issue/CORE-103/vscodetelemetry-remove-legacy-v1-fallback-exporter

Test plan

CI, plus a manual test:

Apply a simple patch to make telemetry export in dev without fenangling configuration (this diff is not in this PR):

diff --git a/vscode/src/services/telemetry-v2.ts b/vscode/src/services/telemetry-v2.ts
index a2857b565..6dcf1e9ad 100644
--- a/vscode/src/services/telemetry-v2.ts
+++ b/vscode/src/services/telemetry-v2.ts
@@ -43,27 +43,11 @@ export function createOrUpdateTelemetryRecorderProvider(

             const initialize = telemetryRecorderProvider === undefined

-            /**
-             * In testing, send events to the mock server.
-             */
-            if (process.env.CODY_TESTING === 'true') {
-                logDebug(debugLogLabel, 'using mock exporter')
-                updateGlobalTelemetryInstances(
-                    new MockServerTelemetryRecorderProvider({
-                        configuration,
-                        clientState,
-                    })
-                )
-            } else if (isExtensionModeDevOrTest) {
-                logDebug(debugLogLabel, 'using no-op exports')
-                updateGlobalTelemetryInstances(defaultNoOpProvider)
-            } else {
                 updateGlobalTelemetryInstances(
                     new TelemetryRecorderProvider(
                         { configuration, auth, clientState },
                     )
                 )
-            }

             /**
              * On first initialization, also record some initial events.

Run the extension:

pnpm install && pnpm build && cd vscode && pnpm run dev

In logs:

█ telemetry-v2 recordEvent: cody.auth/connected:

in the instance:

(cherry picked from commit 717c1d7)

[kalanchan]

Approved by Grace (thread) as emergency patch release to address telemetry issues related to billing metrics.