Add PHP8.4 support by fixing nullable parameter typing, drop support of PHP7.0

[shu-yusa]

From PHP8.4, a parameter that has null as a default value has to be typed as a nullable type: https://www.php.net/manual/en/migration84.deprecated.php
In the code base, the factory methods of Status class have to be fixed to comply with this syntax. On the other hand, as pointed out in #37, nullable type is not available in 7.0 (https://www.php.net/manual/en/migration71.new-features.php), which is supported in this library.
Since the versions 7.0 - 8.0 have reached the EOL (https://www.php.net/supported-versions.php) as of now, I would like to propose dropping support of these versions, and adding support of 8.4. In order not to break users who are using this library in these EOL versions, I think the major version should be lifted.
I also found the library google/auth is not used in any place in the code base. It should be added by library users if necessary, rather than by this library.

[linux-foundation-easycla]

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• ✅ login: shu-yusa / name: Shusaku Yusa (b7e2b06, 5f75c3a, 65c9d6b, 15dadda)

[shu-yusa]

@PNardman @stanley-cheung How's changing like this, and tagging 2.0.0 by the release?

[PNardman]

@PNardman @stanley-cheung How's changing like this, and tagging 2.0.0 by the release?

Plus one would love this!

[mhv666]

Nice I need this as well 💪🏽

[shu-yusa]

@stanley-cheung would you be able to take a look at this PR?

[PNardman]

Everyone is still waiting for this. 🙏

[shu-yusa]

I noticed the README says a request should be sent to https://github.com/grpc/grpc
I created a PR in that repository: grpc/grpc#39199

[shu-yusa]

synced with the update in grpc/grpc#39199. Still waiting for a review

[shu-yusa]

The update has been merged by #40. will close this PR