Add versioned dialects for COCKROACHDB 20, 21

[lukaseder]

CockroachDB 21 ships with a few interesting additions for us, including:

• Native implementation of REGR_INTERCEPT, REGR_R2, REGR_SLOPE: builtins: implement regr_intercept, regr_r2, and regr_slope aggregation functions cockroachdb/cockroach#56296
• Native implementation of REGR_SXX, REGR_SXY, REGR_SYY: builtins: implement regr_sxx, regr_sxy, and regr_syy aggregation functions cockroachdb/cockroach#56585
• Native implementation of REGR_COUNT: builtins: implement regr_count cockroachdb/cockroach#56822
• Native implementation of REGR_AVGX: sql: support regr_avgx() cockroachdb/cockroach#57295
• Native implementation of REGR_AVGY: builtins: implement regr_avgy cockroachdb/cockroach#57583
• Native implementation of COVAR_POP, COVAR_SAMP: builtins: implement covar_pop and covar_samp aggregation functions cockroachdb/cockroach#55707
• String concatenation improvements: sql: allow string concat with any non-array type cockroachdb/cockroach#55611
• CREATE TYPE IF NOT EXISTS: Add support for CREATE TYPE IF NOT EXISTS #11573

To enable / disable these flags, we should start distinguishing between CRDB dialect versions COCKROACHDB_20 and COCKROACHDB_21

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See:
https://www.cockroachlabs.com/docs/releases/v21.1.0-alpha.1.html

[lukaseder]

Can't integration test all of the above yet because of this significant regression in CRDB: cockroachdb/cockroach#61226, which was introduced in alpha.2 and still exists in alpha.3. alpha.1 doesn't seem to have it, but doesn't have all the statistical functions yet