feat: Upgrade Guzzlehttp/psr7 version

[saranshdhingra]

This PR aims to resolve version dependency issues related to older versions(<1.7) for guzzlehttp/psr7 package. See issue #4247 for details.

The use of deprecated methods have been removed and the dependencies have been upgraded.

[dwsupplee]

Looks great, thank you @saranshdhingra! Just one minor comment.

[bshaffer]

These changes look great! However, I suggest we split this into two PRs, one with just the updates to Core, and one with the changes to the other packages which rely on the update to cloud core (namely, Bigquery, Datastore, Spanner, Storage, and Vision because they rely on guzzlehttp/psr7:1.7).

We then merge the PR for Core, cut a new version (v1.43.0), and update the composer.jsons for the above packages to depend on google/cloud-core:^1.43. The packages which do not use the new classes in 1.7 do not need to be updated.

[bshaffer]

These changes look great! However, I suggest we split this into two PRs...

David says we can get away with updating composer.json in the packages I mention above to google/cloud-core:^1.43.0 in this PR (before the release is cut), and release everything at once. So I will leave it up to @dwsupplee / @saranshdhingra to decide whether to split it or not! But you will need to update composer.json for those packages in this PR!

[tjhunkin-inhance]

Is this fix now available?

[saranshdhingra]

Is this fix now available?

Most of the work is done and the dependencies have been updated. We just need to decide on which route to take re: 'splitting' the PRs before releasing.
This should be available soon.

[tjhunkin-inhance]

will this fix be applied to google/cloud-pubsub as well? Because it has the same issue.

[bshaffer]

@tjhunkin-inhance yes, when this rolls out it will address all of our packages.

[tjhunkin-inhance]

Awesome, thanks