Turn custom_sampling_context into span attrs in Celery integration

[sentrivana]

• remove custom_sampling_context and replace the celery_job dict with span attrs
• small fixes in migration guide for rq

[codecov]

❌ 592 Tests Failed:

Tests completed	Failed	Passed	Skipped
19684	592	19092	4424

View the top 2 failed tests by shortest run time

tests.integrations.opentelemetry.test_span_processor test_on_start_transaction

Stack Traces | 0.002s run time

.../integrations/opentelemetry/test_span_processor.py:287: in test_on_start_transaction
    with mock.patch(
.../hostedtoolcache/Python/3.12.7....../x64/lib/python3.12/unittest/mock.py:1463: in __enter__
    original, local = self.get_original()
.../hostedtoolcache/Python/3.12.7....../x64/lib/python3.12/unittest/mock.py:1436: in get_original
    raise AttributeError(
E   AttributeError: <module 'sentry_sdk.integrations.opentelemetry.span_processor' from '.../integrations/opentelemetry/span_processor.py'> does not have the attribute 'start_transaction'

tests.integrations.opentelemetry.test_span_processor test_on_start_transaction

Stack Traces | 0.002s run time

.../integrations/opentelemetry/test_span_processor.py:287: in test_on_start_transaction
    with mock.patch(
.../hostedtoolcache/Python/3.10.15....../x64/lib/python3.10/unittest/mock.py:1447: in __enter__
    original, local = self.get_original()
.../hostedtoolcache/Python/3.10.15....../x64/lib/python3.10/unittest/mock.py:1420: in get_original
    raise AttributeError(
E   AttributeError: <module 'sentry_sdk.integrations.opentelemetry.span_processor' from '.../integrations/opentelemetry/span_processor.py'> does not have the attribute 'start_transaction'

View the full list of 1 ❄️ flaky tests

tests.integrations.opentelemetry.test_span_processor test_on_start_transaction

Flake rate in main: 85.19% (Passed 4 times, Failed 23 times)

Stack Traces | 0.001s run time

.../integrations/opentelemetry/test_span_processor.py:289: in test_on_start_transaction
    fake_start_transaction,
.../hostedtoolcache/Python/3.7.17....../x64/lib/python3.7/unittest/mock.py:1307: in __enter__
    original, local = self.get_original()
.../hostedtoolcache/Python/3.7.17....../x64/lib/python3.7/unittest/mock.py:1281: in get_original
    "%s does not have the attribute %r" % (target, name)
E   AttributeError: <module 'sentry_sdk.integrations.opentelemetry.span_processor' from '.../integrations/opentelemetry/span_processor.py'> does not have the attribute 'start_transaction'

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