Add deprecation warnings to exporter READMEs and update resource detector to be <1.15

[DylanRussell]

Add deprecation warnings to READMEs and update resource detector to be <1.15 as we may make changes to it which break the exporters

[aabmass]

LGTM

$ uv pip compile <(echo 'opentelemetry-resourcedetector-gcp >= 1.5.0dev0, < 1.15')
Resolved 10 packages in 233ms
# This file was autogenerated by uv via the following command:
#    uv pip compile /dev/fd/63
certifi==2026.7.22
    # via requests
charset-normalizer==3.4.9
    # via requests
idna==3.18
    # via requests
opentelemetry-api==1.44.0
    # via
    #   opentelemetry-resourcedetector-gcp
    #   opentelemetry-sdk
    #   opentelemetry-semantic-conventions
opentelemetry-resourcedetector-gcp==1.14.0
    # via -r /dev/fd/63
opentelemetry-sdk==1.44.0
    # via opentelemetry-resourcedetector-gcp
opentelemetry-semantic-conventions==0.65b0
    # via opentelemetry-sdk
requests==2.34.2
    # via opentelemetry-resourcedetector-gcp
typing-extensions==4.16.0
    # via
    #   opentelemetry-api
    #   opentelemetry-sdk
    #   opentelemetry-semantic-conventions
urllib3==2.7.0
    # via requests