Remove deprecated providers

[swade1987]

This issue will track the work required to remove all code not related to the flux_bootstrap_git resource.

Changes required to internal directory

• [ ] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/internal/provider/data_install.go
• [ ] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/internal/provider/data_install_test.go
• [ ] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/internal/provider/data_sync.go
• [ ] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/internal/provider/data_sync_test.go

Changes required to the docs directory:

• [x] Update docs to only include the flux_bootstrap_git resource
• [x] Add an index.md to provide an overview on https://registry.terraform.io/providers/fluxcd/flux/1.2.3

Changes required to the examples directory:

• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/customize-flux
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/data-sources
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/github
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/gitlab
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/gke_github
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/install-resource
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/install
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/multi-env
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/provider
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/tree/main/examples/resources/flux_bootstrap_git

We should update the examples to include just the following scenarios:

• [x] Github bootstrap with SSH (no GPG)
• [x] Github bootstrap with SSH (with GPG)
• [x] Github bootstrap with SSH (customising the bootstrap options)
• [x] Gitlab bootstrap with SSH (no GPG)
• [x] Gitlab bootstrap with SSH (with GPG)
• [x] Helm install

Changes required to the templates directory:

• [x] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/templates/guides/breaking-provider-configuration.md.tmpl
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/templates/guides/extra-syncs.md.tmpl
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/templates/guides/install-helm-release.md.tmpl
• [x] Remove https://github.com/fluxcd/terraform-provider-flux/blob/main/templates/guides/migrating-to-resource.md.tmpl
• [x] Update the guides in the templates directory to match the examples in the section above.

[stefanprodan]

The removal of deprecated providers is scheduled for Flux v2.3 in April.