normalize CQL text support to CQL2 (#2015)

[tomkralidis]

Overview

Normalizes CQL text support for CQL2 only.

Related Issue / discussion

#2015

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[teetin]

Could you also ensure that the appropriate entries to conformance page would be added if this provider is used
I assume those would be (al last,):
http://www.opengis.net/spec/cql2/1.0/conf/cql2-text
http://www.opengis.net/spec/cql2/1.0/conf/basic-cql2
for example QGIS is very strict on these and even with these additions the list is still uncomplete.

Supported filter types (cql2-text and cql2-json) should also be added to:

    LOGGER.debug('Processing filter-lang parameter')
    filter_lang = request.params.get('filter-lang')
    # Currently only cql-text is handled, but it is optional
    if filter_lang not in [None, 'cql-json', 'cql-text', 'cql2-text', 'cql2-json']: