Remove stale net6.0/net7.0 verified snapshots left behind when those TFMs were dropped #5530

Description

test/Sentry.Tests/ still carries six Verify snapshots for target frameworks the repo no longer builds:

test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet6_0.DotNet.verified.txt
test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet6_0.OSX.DotNet.verified.txt
test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet6_0.Windows.DotNet.verified.txt
test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet7_0.DotNet.verified.txt
test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet7_0.OSX.DotNet.verified.txt
test/Sentry.Tests/SentryOptionsTests.Integrations_default_ones_are_properly_registered.DotNet7_0.Windows.DotNet.verified.txt

SentryOptionsTests.Integrations_default_ones_are_properly_registered uses .UniqueForTargetFrameworkAndVersion(), so Verify only ever looks for a snapshot matching a TFM currently being built. net6.0 and net7.0 were dropped in #3807 (cb0a35c), which did not remove the corresponding snapshots — they have been dead files since December 2024. They were last written by #3052.

Nothing references them and no test can select them, so they are safe to delete.

Why it is worth doing

They are actively misleading when working on TFM changes: a find/grep over *.verified.txt suggests those frameworks are still covered, and it is not obvious without checking git history that the files are orphans rather than snapshots for a TFM built somewhere else (Windows-only, mobile, etc.).

Notes

• Spotted while working on Add support for .net 11 preview 7 #5527 / feat: add support for .NET 11 preview 7 #5529 (.NET 11 support). That PR deletes the DotNet8_0 and DotNet9_0 snapshots as part of dropping those TFMs, so this issue is only about the older net6.0/net7.0 leftovers. Doing it separately keeps that diff focused.
• Worth a quick sweep for the same pattern elsewhere rather than just deleting these six — anything matching a TFM not in $(CurrentTfms) and not one of the deliberately platform-specific suffixes (Net4_8, Mono4_0).
• A cheap guard against a recurrence would be a check that every *.<TFM>.verified.txt suffix corresponds to a TFM the repo builds, though that may be more machinery than it is worth.