Asynchronous Connection Closing Causes Panic in HTTP/2

Versions:
* go 1.20+
* go-retryablehttp v0.7.7

**Description:**
I encountered a panic related to HTTP/2 connections while using the `go-retryablehttp` library. The panic message is as follows:

```
panic: id (4) <= evictCount (10)
goroutine 40543 [running]:
vendor/golang.org/x/net/http2/hpack.(*headerFieldTable).idToIndex(0xc000466700, 0xc0007e7320?)
/usr/local/go/src/vendor/golang.org/x/net/http2/hpack/tables.go:118 +0xbd
vendor/golang.org/x/net/http2/hpack.(*headerFieldTable).search(0xc000466700, {{0x13f4875, 0x6}, {0xc000c0a410, 0x10}, 0x0})
/usr/local/go/src/vendor/golang.org/x/net/http2/hpack/tables.go:105 +0xe5
vendor/golang.org/x/net/http2/hpack.(*Encoder).searchTable(0xc000466700, {{0x13f4875, 0x6}, {0xc000c0a410, 0x10}, 0x0})
/usr/local/go/src/vendor/golang.org/x/net/http2/hpack/encode.go:97 +0x85
vendor/golang.org/x/net/http2/hpack.(*Encoder).WriteField(0xc000466700, {{0x13f4875, 0x6}, {0xc000c0a410, 0x10}, 0x0})
/usr/local/go/src/vendor/golang.org/x/net/http2/hpack/encode.go:62 +0x145
net/http.(*http2ClientConn).writeHeader(0xc000baeec0?, {0x13f4875?, 0xffffffffffffffff?}, {0xc000c0a410?, 0xc000743a70?})
/usr/local/go/src/net/http/h2_bundle.go:9177 +0x148
net/http.(*http2ClientConn).encodeHeaders.func3({0xc000baeec0?, 0xc000743a70?}, {0xc000c0a410, 0x10})
/usr/local/go/src/net/http/h2_bundle.go:9111 +0x71
net/http.(*http2ClientConn).encodeHeaders.func1(0xc000a3abe0)
/usr/local/go/src/net/http/h2_bundle.go:9072 +0x627
net/http.(*http2ClientConn).encodeHeaders(0xc000352900, 0xc00069e000, 0x1, {0x0, 0x0}, 0x0)
/usr/local/go/src/net/http/h2_bundle.go:9104 +0x65e
net/http.(*http2clientStream).encodeAndWriteHeaders(0xc000352a80, 0xc00069e000)
/usr/local/go/src/net/http/h2_bundle.go:8578 +0x2ef
net/http.(*http2clientStream).writeRequest(0xc000352a80, 0xc00069e000)
/usr/local/go/src/net/http/h2_bundle.go:8474 +0x5a7
net/http.(*http2clientStream).doRequest(0xc000352a80, 0x0?)
/usr/local/go/src/net/http/h2_bundle.go:8392 +0x18
created by net/http.(*http2ClientConn).RoundTrip in goroutine 40484
```

It's something very complicated to reproduce, it seems that the condition is very specific, perhaps you have more experience with the lib to find a common point for correction.

So far my guess is that this issue is related to the asynchronous closing of connections. According to the discussion in the issue [golang/go#50027](https://github.com/golang/go/issues/50027):

> `CloseIdleConnections` closes all idle connections synchronously.
>
> "Closing" a connection marks the connection as closed (sets `*ClientConn.closed`), so it should never be used for any future requests. It also closes the connection `net.Conn`. However, closing a connection does not block until the connection read loop returns, and the read loop is what removes the `*ClientConn` from the connection pool.
>
> So after `CloseIdleConnections`, some connections may be no longer in use, but still temporarily in the connection pool.
>
> Perhaps closing a connection should block until the connection goroutines have all returned.

Would it be possible to adjust the implementation in `go-retryablehttp` to ensure that all connection goroutines are finished before removing the connection from the pool? This would help prevent panics like the one mentioned above.

Thank you!

More related issues :
- https://github.com/golang/go/issues/43051