fix: Replace unreleased InitialSubscribeRequest initial_cursor field with initial_location

[gcf-owl-bot]

PiperOrigin-RevId: 375562697

Source-Link: googleapis/googleapis@c71311c

Source-Link: https://github.com/googleapis/googleapis-gen/commit/4f125efa59dcd62a71ee7766f14102bfdc012b36

This change was originally a breaking change. I removed the exclamation point ! from the conventional commit message because this feature is unreleased.

[google-cla]

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[parthea]

@googlebot I consent.

[parthea]

Blocked by #148

[dpcollins-google]

owlbot:run

[parthea]

@dpcollins-google Please can you review/approve? The samples failures look unrelated.

The owlbot:run label will signal owl bot to run. Looking at the checks for this PR, it ran successfully.

[parthea]

There is an open pull request to add the owlbot:run label to all repos:
googleapis/repo-automation-bots#1925