opentelemetry-instrumentation-pymemcache: add semconv stability migration support

[SurbhiD404]

Description

Implements semantic convention stability migration support for the pymemcache instrumentation, as tracked in #2453.

Previously, PymemcacheInstrumentor emitted legacy (incubating) database semantic convention attributes unconditionally. This change adds opt-in support for the new stable database semantic convention attributes, controlled by the OTEL_SEMCONV_STABILITY_OPT_IN environment variable.

Attribute mapping

Old (default)	New (database mode)
db.system	db.system.name
db.statement	db.query.text
net.peer.name	server.address
net.peer.port	server.port
net.transport	(no stable equivalent, omitted in new mode)

Fixes #4715

Part of #2453

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Three unit tests were added covering all three semconv modes:

• Default (no opt-in): verifies legacy attributes are present and stable attributes are absent.
• OTEL_SEMCONV_STABILITY_OPT_IN=database: verifies stable attributes are present and legacy attributes are absent.
• OTEL_SEMCONV_STABILITY_OPT_IN=database/dup: verifies both legacy and stable attributes are emitted simultaneously.

To reproduce:

cd instrumentation/opentelemetry-instrumentation-pymemcache
python -m pytest tests/test_pymemcache.py -v

Result:

• 38 tests passed

Does This PR Require a Core Repo Change?

• No.

Checklist

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

Same comment of the cassandra PR about leveraging _semconv _set_db_* helpers plus _set_net_* and _set_http_* ones as well.

Please does not open more than one PR at a time, wait to get approval from one so you can apply what you learned in the process.