opentelemetry.tracer crashes with TypeError in transaction with high number of spans

**Describe the bug**:

Using opentelemetry bridge crashes with `TypeError: 'NoneType' object does not support item assignment` exception when number of spans in transaction is higher than `ELASTIC_APM_TRANSACTION_MAX_SPANS`

```
File ~/.pyenv/versions/3.11.11/envs/test/lib/python3.11/site-packages/elasticapm/contrib/opentelemetry/trace.py:169, in Tracer.start_span(self, name, context, kind, attributes, links, start_time, record_exception, set_status_on_exception)
167 span.set_attributes(attributes)
168 spankind = get_span_kind(kind)
--> 169 elastic_span.context["otel_spankind"] = spankind
171 return span
````
.
Debugger shows that `elastic_span` inside is `DroppedSpan`. Increasing nr of `ELASTIC_APM_TRANSACTION_MAX_SPANS` "fixes" problem as shown in code below.

**To Reproduce**

```python
from elasticapm.contrib.opentelemetry import trace
from elasticapm import Client, capture_span

import os

# Reduce number of spans required to reproduce the issue
ELASTIC_APM_TRANSACTION_MAX_SPANS = 10
STEPS_ABOVE = ELASTIC_APM_TRANSACTION_MAX_SPANS + 1
STEPS_BELLOW = ELASTIC_APM_TRANSACTION_MAX_SPANS - 1
os.environ["ELASTIC_APM_TRANSACTION_MAX_SPANS"] = str(ELASTIC_APM_TRANSACTION_MAX_SPANS)

apm_client = Client()
tracer = trace.get_tracer(__name__)

# Simulate deep nested spans
def capture_span_elasticapm(steps=10):
if steps == 0:
return

with capture_span():
capture_span_elasticapm(steps-1)

def capture_span_opentelemetry(steps=10):
if steps == 0:
return

with tracer.start_as_current_span("test"):
capture_span_opentelemetry(steps-1)

# TEST

# Works - Expected
transaction = apm_client.begin_transaction('processors')
capture_span_elasticapm(STEPS_BELLOW)
apm_client.end_transaction('myapp.billing_process')

# Works - Expected
transaction = apm_client.begin_transaction('processors')
capture_span_opentelemetry(STEPS_BELLOW)
apm_client.end_transaction('myapp.billing_process')

# Works - Expected
transaction = apm_client.begin_transaction('processors')
capture_span_elasticapm(STEPS_ABOVE)
apm_client.end_transaction('myapp.billing_process')

# Crashes - Unexpected
# TypeError: 'NoneType' object does not support item assignment
transaction = apm_client.begin_transaction('processors')
capture_span_opentelemetry(STEPS_ABOVE)
apm_client.end_transaction('myapp.billing_process')
```

**Environment (please complete the following information)**
- OS: [e.g. Linux] Linux Mint 22.1 Cinnamon
- Python version: `3.11.11`
- Framework and version [e.g. Django 2.1]: N/A
- APM Server version: N/A
- Agent version: `elastic-apm==6.23.0`

Empieza en elasticapm/contrib/opentelemetry/trace.py, en la ruta Tracer.start_span alrededor de la línea 169, y luego ejecuta la reproducción proporcionada con spans anidados por encima de ELASTIC_APM_TRANSACTION_MAX_SPANS. Se considera terminado cuando la transacción de OpenTelemetry se completa sin el fallo de asignación de elementos de tipo NoneType cuando los spans superan el límite.

Escrito por el modelo de indexación a partir del texto del issue.