feat: add VERSION file for fluentbit image

[cw-Guo]

What this PR does / why we need it:

improve image build process

Which issue(s) this PR fixes:

Fixes #

Does this PR introduced a user-facing change?

Additional documentation, usage docs, etc.:

[cw-Guo]

@benjaminhuo I added a VERSION file to specify the fluent-bit image version so that we can avoid updating multiple places when bumpping the version.

Also removed the docker_tag_version from the workflow input, it can be read from the VERSION file.

See tests here: https://github.com/cw-Guo/fluent-operator/actions/runs/12593318269

https://github.com/cw-Guo/fluent-operator/actions/runs/12593457069/job/35099495400#step:7:183

[benjaminhuo]

cc @wanjunlei @wenchajun

[benjaminhuo]

@benjaminhuo I added a VERSION file to specify the fluent-bit image version so that we can avoid updating multiple places when bumpping the version.

Also removed the docker_tag_version from the workflow input, it can be read from the VERSION file.

See tests here: https://github.com/cw-Guo/fluent-operator/actions/runs/12593318269

https://github.com/cw-Guo/fluent-operator/actions/runs/12593457069/job/35099495400#step:7:183

If you search 3.1.8 globally in the code, you may found other places that need to be changed as well @cw-Guo