Update readme with install instrctions (oapi-codegen#1652)

There are go.mod module name mismatches now that we've moved repos. Mention
this in the readme, and inform that v2.3.0 is the first working version in the
new location.

chore: deprecate v1 module

docs: add Sponsors section

Plus DevZero, our first official sponsor 👏

Fix: Ensure external refs are propagated to generated code

As noted in oapi-codegen#1378, there are cases where a complex set of `$ref`s
between multiple files can lead to broken generated code, which does not
correctly import the package that has been prepared for the external
reference.

We can handle this by looking up any references, where there is a `.Ref`
passed into the type, and then iterate through relevant children.

This requires we handle the updating in-place for these by using a bit
of pointer + indexing fun.

This also adds a relevant test case to validate the fix.

Closes oapi-codegen#1378.

Bump module import path to v2

Ahead of release, we need to update to the v2 package structure, which
requires quite a few changes across packages.

Add missing types to deprecation URLs

Fix: Ensure deprecations are parsed correctly

It appears that GoDoc does not parse these deprecation warnings as a
deprecation unless there is an explicit blank comment between lines, or
it is the start of a doc comment.

Add deprecation notice for `testutil`

Bump Iris to remove Go 1.21 dependency

As part of the upgrade to Go 1.21, the Go toolchain now requires the
`go` directive to match the maximum Go version in use in dependencies.

This leads to any transitive dependency on Iris to result in a
requirement of the consuming project moving to Go 1.21.

We can upgrade Iris to a version that has downgraded the Go 1.21
requirement.

We will pin to the latest commit on HEAD until v12.2.6 is released.

Closes oapi-codegen#1221.

Allow overriding the binary version when VCS not available (oapi-code…

…gen#1163)

As mentioned in oapi-codegen#1046, for environments, like Nix, that build without a
Version Control System (VCS) available, we output `(devel)` as the
version of oapi-codegen even when it's building from source from a Git
tag.

We can therefore provide a `noVCSVersionOverride` build-time flag to
allow us to override this with an arbitrary string.

Closes oapi-codegen#1046.

Co-authored-by: Jamie Tanna <jamie@jamietanna.co.uk>