Bump dependencies to support guzzle v6

[phil-davis]

This gets composer.lock having:

            "name": "guzzlehttp/guzzle",
            "version": "6.5.5",

https://github.com/kunalvarma05/dropbox-php-sdk/blob/master/composer.json only mentions guzzle v6 so far.

And https://github.com/Hemant-Mann/flysystem-dropbox/blob/master/composer.json requires:

"kunalvarma05/dropbox-php-sdk": "^0.2.0"

So it looks like we can get to guzzle 6 easily, but guzzle 7 will need investigation.

Maybe guzzle v6 here will work OK with core guzzle 7 anyway? The separate composer.lock of core and the app should find the right versions of dependencies as code is called.

[phil-davis]

Note: CI does not run the unit tests. IMO that needs a test dropbox set up somehow/somewhere. If this goes ahead, then manual testing will be required.

[JammingBen]

I did some testing. I was able to connect to Dropbox, list, rename & upload files. All other actions I tried (download, open) failed - but so they do with master.

[JammingBen]

Also we should raise the minimum OC version to 10.9 with this PR.

[phil-davis]

Also we should raise the minimum OC version to 10.9 with this PR.

Done.

We can leave this PR until we know all of the apps that need touching, and can make a decision about merging guzzle 7 in core...

[phil-davis]

Note: update to Guzzle 7 in core is waiting until after 10.9.0 release. I converted this PR to Draft for now.

[phil-davis]

Rebased and got the dependencies up-to-date. Now composer.lock has guzzle 7.4.4 7.4.5

Note: this app has no tests that run in automated CI, so it will need manual testing after core goes to guzzle 7.4.5

[phil-davis]

I rebased just now. IMO we merge this, and put this app in the list of things to be released along with core 10.11

@JammingBen @janackermann

[phil-davis]

I made release issue #133